2004 Petroleum Science 1No.2 Complex Exploration Techniques for the Low-permeability Lithologic Gas Pool in the Upper Paleozoic of ordos Basin Fu Jinhua, Xi Shengli, Liu Xinshe and Sun Liuyi Ite of Exploration and Development, Changqing Oilfield Branch company, Petro China, XI'an, Shaanxi 710021, China) Abstract: The Ordos basin is a stable craton whose late Paleozoic undergoes two sedimentary stages: from the middle- late Carboniferous offshore plain to the Permian continental river and lake delta. Sandstones in delta plain channels, delta-front river mouth bars and tidal channels are well developed. The sandstones are distributed on or between the genetic source rocks, forming good gas source conditions with widespread subtle lithologic gas pools of low porosity, low permeability, low pressure and low abundance. In recent years, a series of experiments has been done, aimed at overcoming difficulties in the exploration of lithologic gas pools. A set of exploration techniques, focusing on geological appraisal, seismic exploration, accurate logging evaluation and interpretation, well testing fracturing, has been developed to guide the exploration into the upper Paleozoic in the basin, leading to the discoveries of four large gas fields: Sulige, Yulin, Wushenqi and Mizhi Key words: Ordos Basin, upper Paleozoic, lithologic gas pool, seismic exploration, accurate logging evaluation, exploration techniques 1 Introduction 2. Features of sandstone lithologic gas pools and difficulties in their exploration As the second largest petroliferous basin in our country, the Ordos basin has an upper Paleozoic that has 2.1 Lithologic traps experienced gas exploration area of over 20 x 10'km2 of The Ordos basin is a stable craton whose layers are the. Gas reserves found in this area are mainly from basically horizontal, with a westward monocline which has subtle lithologic gas pools. Gas exploration of the a slope of less than 0.5o. The wide area in the basin lacks Upper Paleozoic has passed a hard and flexural way just secondary structural zones and regional structures that can because of the technical difficulties. From the 50s to the form structural reservoirs for oil accumulation and mid-90s,gas exploration was focused on searching for enrichment. Therefore, reservoirs in the basin are mainly structural reservoirs. The lack of knowledge about the non-anticline subtle ones instead of structural reservoirs plexities of sandstone lithologic gas reservoirs and The fault-fold zones on the western edge is a typical of corresponding technologies prevented our imbricated fold structure, with blocks in the south-north exploration from making great breakthroughs. Ever direction and belts in the east-west direction, they often since the gth"Five-Year Plan"period, with a better develop into fault- barrier reservoirs. The severe damage of understanding of geology, we have developed a series the local structures has limited their areas, and therefore of key technologies aimed at the upper Paleozoic we have found only small reservoirs in these structures sandstone lithologic gas pools, whose exploration has The Yimeng uplift zone in the north, an overlapping now entered into a new era. Large gas fields such as sedimentary area of the late Paleozoic and mesozoic, is a Yulin, Wushenqi and Sulige have been found one by small unconformity reservoir because of its poor gas one,which shows a brilliant prospecting future for the source. The Jinxi flexural fold zone in the east and Weibei upper Paleozoic sandstone lithologic gas pools. Such uplift in the south are a relatively gentle fold area with well technologies as high-resolution seismic data acquisition developed anticline structural reservoirs but with poor oil and processing, prediction of the reservoir thickness, enrichment. Most areas in the basin, which are low- accurate reservoir interpretation, gas layer identificatio permeability lithologic traps because of the stable nd fracturing alteration provide technical support for subsidence and marine-continental sedimentary systems gas exploration not only in this basin, but also in others have become the main target of gas exploration in spite of with pools ofthe same type the difficulties in trap identification o1994-2007ChinaAcademicJournalElectronicpUblishingHouse.Allrightsreservedhttp://www.cnki.net
2004 Petroleum Science Vol.1 No.2 Complex Exploration Techniques for the Low-permeability Lithologic Gas Pool in the Upper Paleozoic of Ordos Basin Fu Jinhua, Xi Shengli, Liu Xinshe and Sun Liuyi (Research Institute of Exploration and Development, Changqing Oilfield Branch company, PetroChina, Xi'an, Shaanxi 710021, China) Received January 30, 2004 Abstract: The Ordos basin is a stable craton whose late Paleozoic undergoes two sedimentary stages: from the middlelate Carboniferous offshore plain to the Permian continental river and lake delta. Sandstones in delta plain channels, delta-front river mouth bars and tidal channels are well developed. The sandstones are distributed on or between the genetic source rocks, forming good gas source conditions with widespread subtle lithologic gas pools of low porosity, low permeability, low pressure and low abundance. In recent years, a series of experiments has been done, aimed at overcoming difficulties in the exploration of lithologic gas pools. A set of exploration techniques, focusing on geological appraisal, seismic exploration, accurate logging evaluation and interpretation, well testing fracturing, has been developed to guide the exploration into the upper Paleozoic in the basin, leading to the discoveries of four large gas fields: Sulige, Yulin, Wushenqi and Mizhi. Key words: Ordos Basin, upper Paleozoic, lithologic gas pool, seismic exploration, accurate logging evaluation, exploration techniques 1. Introduction As the second largest petroliferous basin in our country, the Ordos basin has an upper Paleozoic that has experienced gas exploration area of over 20 × 104 km2 of the. Gas reserves found in this area are mainly from subtle lithologic gas pools. Gas exploration of the Upper Paleozoic has passed a hard and flexural way just because of the technical difficulties. From the 50s to the mid-90s, gas exploration was focused on searching for structural reservoirs. The lack of knowledge about the complexities of sandstone lithologic gas reservoirs and of corresponding technologies prevented our exploration from making great breakthroughs. Ever since the 9th "Five-Year Plan" period, with a better understanding of geology, we have developed a series of key technologies aimed at the upper Paleozoic sandstone lithologic gas pools, whose exploration has now entered into a new era. Large gas fields such as Yulin, Wushenqi and Sulige have been found one by one, which shows a brilliant prospecting future for the upper Paleozoic sandstone lithologic gas pools. Such technologies as high-resolution seismic data acquisition and processing, prediction of the reservoir thickness, accurate reservoir interpretation, gas layer identification and fracturing alteration provide technical support for gas exploration not only in this basin, but also in others with pools of the same type. 2. Features of sandstone lithologic gas pools and difficulties in their exploration 2.1 Lithologic traps The Ordos basin is a stable craton whose layers are basically horizontal, with a westward monocline which has a slope of less than 0.5°. The wide area in the basin lacks secondary structural zones and regional structures that can form structural reservoirs for oil accumulation and enrichment. Therefore, reservoirs in the basin are mainly non-anticline subtle ones, instead of structural reservoirs. The fault-fold zones on the western edge is a typical imbricated fold structure, with blocks in the south-north direction and belts in the east-west direction, they often develop into fault-barrier reservoirs. The severe damage of the local structures has limited their areas, and therefore, we have found only small reservoirs in these structures. The Yimeng uplift zone in the north, an overlapping sedimentary area of the late Paleozoic and Mesozoic, is a small unconformity reservoir because of its poor gas source. The Jinxi flexural fold zone in the east and Weibei uplift in the south are a relatively gentle fold area with well developed anticline structural reservoirs but with poor oil enrichment. Most areas in the basin, which are lowpermeability lithologic traps because of the stable subsidence and marine-continental sedimentary systems, have become the main target of gas exploration in spite of the difficulties in trap identification
l12 Petroleum science 2004 2.3 Low-pressure systems with a high content of 2.2 Low-permeability, low-porosity but thin reservoirs methane and low oil productivity The upper Paleozoic is a set of continental-marine The basin is dominated by low-pressure gas pools, with interbedding coal-bearing series, which can be classified coefficients between 0.80-0.95. These pressure systems, into six groups, namely Benxi, Taiyuan, Shan complicated by the barrier of mue one an Shangshihezi, Xiashihezi and Shiqianfeng. The late in blocks in the horizontal direction and layers in the vertical Paleozoic consists of five sedimentary systems including direction. On a certain plane, the same layers within one gas continental shelves, barrier-lagoons, tidal flats, deltas and field can have either high-pressure or low-pressure pools. In rivers. The gas-bearing groups are mainly the Shan-l and the vertical direction, the pressure coefficients tend to Shan-2 sections in the Shanxi group, the He-8 member in increase from the shallow layer to the deep layer he lower shih group, and th The NG in this area is mainly composed of methane Shiqianfeng group. The upper Paleozoic sandstone with low contents of heavy hydrocarbon and condensate hologic reservoirs in these areas are characterized by low oil. The content of methane ranges from 73. 6% to 99.4%, averaging 93.15%, with relative densities between 0.5471 u m"]. The thickness of sandstones ranges from several to d 0. 7427 and cO, content of about 1. 13%. On a certain dozens of meters, with great horizontal changes. The main plane, NG components are closely related to the maturity body of the sandstones extends from the south to the north of the source rock: high maturity usually indicates a high in the horizontal direction and overlaps with each other in content of methane. The side and bottom water in the gas the vertical direction, presenting features of braided river pool is not obvious. The well productivity averages sedimentation. The average size of the sandstones is large. (3-6)x 10m/d. Most wells cannot offer an industrial They contain mainly coarse and medium-sized grains of output without fracturing gravel, with some fine or granular grains in between. experiencing furious anadiagenesis and inhomogene The residual primary intergranular pores take a dominant 92040608010km position while the secondary and crevice ones are also noticeable. Diagenetic clay minerals are well developed The reservoirs are usually vulnerable to damage, with well developed scaly or micro throats which are sensitive to stress changes. The complex and loose surface relief of the basin, mainly loess and desert, which contributes to much reduction of wave energy and weak reflection of seismic information, brings about much difficulty in reservoir prediction through seismic technologies. On the other har these features impose special difficulties to the logging evaluation of the low porosity and permeability reservoirs First, with oil reservoirs usually making up less than 10% in the total volume of rock, there is little logging response from the oil component, thus greatly reducing the resolution of logging materials to the pore fluids. Second because of the strong anadiagenesis and micro and macro inhomogeneities, the log interpretation model is influenced by complex and changeable factors, and therefore becomes less applicable. In addition, the affect of non- ig. 1 The relationship between gas-production index and geological factors like reservoir protection and distribution of gas pools in the upper Paleozoic reconstruction on oil productivity makes it more difficult to identify and evaluate the industrious production layers, 3. Enrichment patterns of upper paleozoic low-production layers and dry layers. Finally, the lithologic gas poo stratigraphic permeability of the rock does not have a linear relationship with the ground permeability, but 3.1 The generative center controls the distribution of lated to diversified factors such as mineral components gas pools and grain sizes The generative center effectively controls the o1994-2007chinaAcademicjOurnalElectronicPublishinghOuse.Allrightsreservedhttp:/www.cnki.net
112 Petroleum Science 2004 2.2 Low-permeability, low-porosity but thin reservoirs The upper Paleozoic is a set of continental-marine interbedding coal-bearing series, which can be classified into six groups, namely Benxi, Taiyuan, Shanxi, Shangshihezi, Xiashihezi and Shiqianfeng. The late Paleozoic consists of five sedimentary systems including continental shelves, barrier-lagoons, tidal flats, deltas and rivers. The gas-bearing groups are mainly the Shan-1 and Shan-2 sections in the Shanxi group, the He-8 member in the lower Shihezi group, and the Qian-5 member in the Shiqianfeng group. The upper Paleozoic sandstone lithologic reservoirs in these areas are characterized by low porosity (4%~10%) and low permeability [(0.1~5)× 10-3 u m2 ]. The thickness of sandstones ranges from several to dozens of meters, with great horizontal changes. The main body of the sandstones extends from the south to the north in the horizontal direction and overlaps with each other in the vertical direction, presenting features of braided river sedimentation. The average size of the sandstones is large. They contain mainly coarse and medium-sized grains of gravel, with some fine or granular grains in between, experiencing furious anadiagenesis and inhomogeneity. The residual primary intergranular pores take a dominant position while the secondary and crevice ones are also noticeable. Diagenetic clay minerals are well developed. The reservoirs are usually vulnerable to damage, with well developed scaly or micro throats which are sensitive to stress changes. The complex and loose surface relief of the basin, mainly loess and desert, which contributes to much reduction of wave energy and weak reflection of seismic information, brings about much difficulty in reservoir prediction through seismic technologies. On the other hand, these features impose special difficulties to the logging evaluation of the low porosity and permeability reservoirs. First, with oil reservoirs usually making up less than 10% in the total volume of rock, there is little logging response from the oil component, thus greatly reducing the resolution of logging materials to the pore fluids. Second, because of the strong anadiagenesis and micro and macro inhomogeneities, the log interpretation model is influenced by complex and changeable factors, and therefore, becomes less applicable. In addition, the affect of nongeological factors like reservoir protection and reconstruction on oil productivity makes it more difficult to identify and evaluate the industrious production layers, low-production layers and dry layers. Finally, the stratigraphic permeability of the rock does not have a linear relationship with the ground permeability, but is related to diversified factors such as mineral components and grain sizes. 2.3 Low-pressure systems with a high content of methane and low oil productivity The basin is dominated by low-pressure gas pools, with coefficients between 0.80~0.95. These pressure systems, complicated by the barrier of mudstone and tight sand, fall in blocks in the horizontal direction and layers in the vertical direction. On a certain plane, the same layers within one gas field can have either high-pressure or low-pressure pools. In the vertical direction, the pressure coefficients tend to increase from the shallow layer to the deep layer. The NG in this area is mainly composed of methane, with low contents of heavy hydrocarbon and condensate oil. The content of methane ranges from 73.6% to 99.4%, averaging 93.15%, with relative densities between 0.5471 and 0.7427 and CO2 content of about 1.13%. On a certain plane, NG components are closely related to the maturity of the source rock: high maturity usually indicates a high content of methane. The side and bottom water in the gas pool is not obvious. The well productivity averages (3~6)× 104m 3 /d. Most wells cannot offer an industrial output without fracturing. Fig. 1 The relationship between gas-production index and distribution of gas pools in the upper Paleozoic 3. Enrichment patterns of upper Paleozoic lithologic gas pools 3.1 The generative center controls the distribution of gas pools The generative center effectively controls the
VoL I No Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin distribution of gas pools. Seen from the features of suitable for the enrichment of natural gas hydrocarbon generation and expulsion in the source rock, the Mizhi-Yanchang area in the east of the basin 3.3 The relative high-permeability zones are the main has accumulation intensities of 40-60x10m/km, the geological factor for large gas pool formation central part of the basin having accumulation intensities Our analysis of 6, 362 samples from the upper Paleozoic of 24-35x 10m/km, while most areas in the north, reservoir layers shows that there are some relatively high- east and west of the basin have of 5-15 x 10m/km". porosity and -permeability reservoirs against the general The found large gas fields are all located in the middle background of low porosity and permeability(Fig. 2) and eastern parts of basin where the accumulation Samples with porosity above 10% make up 13.1%,while intensities exceeds 18x10.m/km(Fig. 1) those with permeability above 1x10 u m account for I In the vertical direction, the size of a gas reservoir at With present techniques, such reservoirs can produce middle different layers is also controlled by the generative or high yields after fracturing and become"sweet cake"areas center of source rocks. The Shanxi and Xiashihezi for the discovery of large gas fields. The exploration practice groups, neighboring the center and equipped with in Yuling and Sulige gas pools shows that the distribution of adequate sources, contain large gas-containing areas and high permeability reservoirs is controlled mainly by large gas reservoirs and have become the major sedimentary fades and belts, with high permeability sand production sections of the upper Paleozoic in the basin. bodies formed in the high-energy stream surroundings in the On the other hand, The Shangshihezi and Shiqianfeng form of delta plain distributary channels and delta front groups, far away from the source rocks, have only local distributary channels ( Table 1). The lithology of these and small gas pools sandstones shows a high content of quartz, good sorting large diameters of grains. The sandstones are superim 3.2 Delta sand bodies are main places for reservoir with each other, with thick single bodies. The pores are enrichment mainly secondary and intergranular, often with medium or The later paleozoic littoral delta and river-controlled larges diameters. The throats are mostly micro or thin sedimentary systems dominate the Ordos basin in geologic times. Exploration practices in the basin show that natural gas is mainly distributed in the sedimentary areas of delta channels and delta front underwater channels or transitional areas. The four discovered major gas enrichment zones, including Suligemiao, Wushenqi, Yulin and Shenmu-Zizhou are distributed from the south to the north in the form of parallel bars, whose strikes are similar to that of the main delta systems in the north in the permian. River -controlled deltas are the dominant sedimentary bodies in the basin. Distribution range of porosity after core analysis characterized by delta plain channels and delta front Fig. 2 Upper Paleozoic reservoir porosity distribution underwater channels with thick sand bodies. which is frequency in the Ordos basin Tablet Character of sandstones of dififerent sediment micro-facies Physical properties Porosity (0) Permeability(×103um) Min-Max Sediment facies type Average Average Distributary channel sandstones(586) 13-1996 0.0027~561 River mouth bar sandstones(237) 06-196 0.01~18 31 7.16 Front sheeting sandstones(46) 108-782 0.01~0 5.01 C1994-2007chinaAcademicJournaleLectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
Vol.1 No.2 Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin 113 distribution of gas pools. Seen from the features of hydrocarbon generation and expulsion in the source rock, the Mizhi-Yanchang area in the east of the basin has accumulation intensities of 40~60×108m 3 /km2 , the central part of the basin having accumulation intensities of 24~35× 108m 3 /km2 , while most areas in the north, east and west of the basin have of 5~15 × 108m 3 /km2 . The found large gas fields are all located in the middle and eastern parts of basin where the accumulation intensities exceeds 18×108m 3 /km (Fig. 1). In the vertical direction, the size of a gas reservoir at different layers is also controlled by the generative center of source rocks. The Shanxi and Xiashihezi groups, neighboring the center and equipped with adequate sources, contain large gas-containing areas and large gas reservoirs and have become the major production sections of the upper Paleozoic in the basin. On the other hand, The Shangshihezi and Shiqianfeng groups, far away from the source rocks, have only local and small gas pools. 3.2 Delta sand bodies are main places for reservoir enrichment The later Paleozoic littoral delta and river-controlled sedimentary systems dominate the Ordos basin in geologic times. Exploration practices in the basin show that natural gas is mainly distributed in the sedimentary areas of delta channels and delta front underwater channels or transitional areas. The four discovered major gas enrichment zones, including Suligemiao, Wushenqi, Yulin and Shenmu-Zizhou, are distributed from the south to the north in the form of parallel bars, whose strikes are similar to that of the main delta systems in the north in the Permian. River-controlled deltas are the dominant sedimentary bodies in the basin, characterized by delta plain channels and delta front underwater channels with thick sand bodies, which is suitable for the enrichment of natural gas. 3.3 The relative high-permeability zones are the main geological factor for large gas pool formation Our analysis of 6,362 samples from the upper Paleozoic reservoir layers shows that there are some relatively highporosity and -permeability reservoirs against the general background of low porosity and permeability (Fig. 2). Samples with porosity above 10% make up 13.1%, while those with permeability above 1×10-3 μ m 2 account for 15%. With present techniques, such reservoirs can produce middle or high yields after fracturing and become "sweet cake" areas for the discovery of large gas fields. The exploration practice in Yuling and Sulige gas pools shows that the distribution of high permeability reservoirs is controlled mainly by sedimentary fades and belts, with high permeability sand bodies formed in the high-energy stream surroundings in the form of delta plain distributary channels and delta front distributary channels (Table 1). The lithology of these sandstones shows a high content of quartz, good sorting and large diameters of grains. The sandstones are superimposed with each other, with thick single bodies. The pores are mainly secondary and intergranular, often with medium or larges diameters. The throats are mostly micro or thin. Distribution range of porosity after core analysis Fig. 2 Upper Paleozoic reservoir porosity distribution frequency in the Ordos basin Tablet Character of sandstones of different sediment micro-facies Physical properties Sediment facies type Distributary channel sandstones (586) River mouth bar sandstones (237) Front sheeting sandstones (46) Porosity (%) Min,—Max. Average Permeability( × 10-3 μm 2 ) Min.—Max. Average 13~19.96 0.0027~561 8.02 2.17 0.06~19.6 0.01 ~ 18.31 7.16 0.81 1.08~7.82 0.01~0.82 5.01 0.47
Petroleum Science HD402 HDI-2 HDI 050 4050 4100 he sandstone 15 Fig. l Profile section of the Carboniferous reservoir in Hade 4 Oilfield south to north and even pinches out. The biggest The saturation pressure is far below the stratigraphic thickness of a single well drilled is 29.0m. The Donghe pressure, and the gap between the stratigraphic pressure sandstone trap is a compound trap controlled by the and the saturation pressure is large, so it belongs with low-amplitude anticline and the Donghe sandstone an unsaturated oil reservoi pinch-out line, and consists of two relativel independent crests, namely HD4 crest and HD1-2 crest, 2. Key technical problem and philosophy of (shown in Fig. 2). The crest is at an altitude of-4090.0m, research the structural trap amplitude being 34.5m. The trap is mall, but its oil-containing area is 136 km, relatively The exploration of the Donghe sandstone subtle big. The type of the reservoir is of chiefly medium and reservoir in Hade 4 oil field is confronted with three key high porosity and permeability. The porosity is between technical problems: The first is the accurate 12%-24%, averaged 15.65%. The permeability zone discrimination of the Donghe sandstone pinch-out lines the second is the variable speed mapping of low- amplitude structure; and the third is the establishment of the tilting water-oil contact and the analysis of its genesis for an accurate calculation of its oil containing range There are some major problems arising in both the study of accurate structure and the horizontal prediction of the reservoir as follows: (1) The structural amplitude of Hade 4 oilfield is below 34m, the burial depth of the target layer measures over 5,050m and is located under the igneous velocity anomaly body. No obvious structure appears in the to map; only a broad and gentle nose uplift can be seen causing great difficulty in speed research and variable (2) The project is located in an eluvial soil-cover area in a big desert, and the shooting and reception conditions are relatively poor. In addition, the target layer is also buried deep, and the thickness of the Fig. 2 Top tectonic map of the Carboniferous thin reservoir is small (the biggest thickness is below 30m) sand layer and the Donghe sandstone reservoir so, in the condition of existing seismic data resolution, in Hade 4 Oilfield it is difficult to directly recognize the Donghe sandstone <1-2000×103μum2, averaged126.51×103μm2. The reservoir and its pinch- out point resistivity of the reservoir is low, so a reservoir of an (3)The Donghe sandstone overlapped the formatio ly resistivity is developed the lowest of the Silurian deposited sand-shale alternating layers at ivity being 0. 50Q2m, consistent with the layer of angular unconformity hickness and physical water. The reservoir has a pressure coefficient of property itself has the feature of a horizontal 1. 10-1. 11, and belongs to a normal pressure system. heterogeneity. Additionally, the attitude of the C1994-2007ChinaAcademicJournalElectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
36 Petroleum Science 2004 Fig.1 Profile section of the Carboniferous reservoir in Hade 4 Oilfield south to north and even pinches out. The biggest thickness of a single well drilled is 29.0m. The Donghe sandstone trap is a compound trap controlled by the low-amplitude anticline and the Donghe sandstone pinch-out line, and consists of two relatively independent crests, namely HD4 crest and HD1-2 crest, (shown in Fig.2). The crest is at an altitude of-4090.0m, the structural trap amplitude being 34.5m. The trap is small, but its oil-containing area is 136 km2 , relatively big. The type of the reservoir is of chiefly medium and high porosity and permeability. The porosity is between 12%~24%, averaged 15.65%. The permeability zone is Fig. 2 Top tectonic map of the Carboniferous thin sand layer and the Donghe sandstone reservoir in Hade 4 Oilfield <1~2000×10-3μm 2 , averaged 126.51 × 10-3 μ m 2 . The resistivity of the reservoir is low, so a reservoir of an extremely low resistivity is developed, the lowest resistivity being 0.50Ωm, consistent with the layer of water. The reservoir has a pressure coefficient of 1.10~1.11, and belongs to a normal pressure system. The saturation pressure is far below the stratigraphic pressure, and the gap between the stratigraphic pressure and the saturation pressure is large, so it belongs with an unsaturated oil reservoir. 2. Key technical problem and philosophy of research The exploration of the Donghe sandstone subtle reservoir in Hade 4 oil field is confronted with three key technical problems: The first is the accurate discrimination of the Donghe sandstone pinch-out lines; the second is the variable speed mapping of lowamplitude structure; and the third is the establishment of the tilting water-oil contact and the analysis of its genesis for an accurate calculation of its oil containing range. There are some major problems arising in both the study of accurate structure and the horizontal prediction of the reservoir as follows: (1) The structural amplitude of Hade 4 oilfield is below 34m, the burial depth of the target layer measures over 5,050m and is located under the igneous velocity anomaly body. No obvious structure appears in the T0 map; only a broad and gentle nose uplift can be seen, causing great difficulty in speed research and variablespeed spatial correction. (2) The project is located in an eluvial soil-cover area in a big desert, and the shooting and reception conditions are relatively poor. In addition, the target layer is also buried deep, and the thickness of the reservoir is small (the biggest thickness is below 30m), so, in the condition of existing seismic data resolution, it is difficult to directly recognize the Donghe sandstone reservoir and its pinch-out point. (3) The Donghe sandstone overlapped the formation of the Silurian deposited sand-shale alternating layers at angular unconformity. Its thickness and physical property itself has the feature of a horizontal heterogeneity. Additionally, the attitude of the
VoL I NO.2 Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin to tell oil reservoirs from non-reservoir layers that are From relationship between the real rock gas saturation similar in wave impedance. To the deep consolidated and Passions ratio in Yulin and Sulige area we can learn rock, sandstones and mudstones, whether it be oil that the Passion s ratio of tight sandstones and mudstones reservoirs or non-reservoir layers, may present similar remains above 0. 180 in natural weathering, while that of wave impedance. But their differences in matrix gas-bearing sandstones ranges from 0. 13 to 0. 17 under the omponents and pore structures often lead to major same condition. It can be safely concluded, therefore, that differences in Poisson's ratios. So, the AvO analysis the AvO analysis technique, referring to the the Passion's technique may, to a certain degree, avoid errors in ratio, is practicable to predict the gas saturation in the ismic interpretation. A combination of this method Upper Paleozoic sandstone reservoirs. Recent years have with waveshape analysis of conventional seismic seen its wide use in Yulin and Sulige areas, with a su sections and lithologic reversion can make lithology ratio of gas-bearing detection over 80% in the explor identification and gas saturation detection more reliable perIod Denoising eliminary processing the seismic target Frequency increase Geological Match information of the target Caliper correction of the logging target Time sampling Comprehensive calibration of the reservoir Logging pretation o thickness Automatic classification of seismic facies. Analysis of reflected wave-group features Synthetic sonic logging iterpretation of constrained-inversion Logging technology information seryretation of Rarefaction pulse inversion thickness Simulate annealing inversion AVO analysis Absorbing coefficient calculation Seismic multi-parameter inversion Pattern identification Chart 1 Flow chart of techniques for horizontal prediction of upper Paleozoic reservoirs S1994-2007chinaAcademicJournalElectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
Vol.1 No.2 Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin 115 to tell oil reservoirs from non-reservoir layers that are similar in wave impedance. To the deep consolidated rock, sandstones and mudstones, whether it be oil reservoirs or non-reservoir layers, may present similar wave impedance. But their differences in matrix components and pore structures often lead to major differences in Poisson's ratios. So, the AVO analysis technique may, to a certain degree, avoid errors in seismic interpretation. A combination of this method with waveshape analysis of conventional seismic sections and lithologic reversion can make lithology identification and gas saturation detection more reliable. From relationship between the real rock gas saturation and Passion's ratio in Yulin and Sulige area we can learn that the Passion's ratio of tight sandstones and mudstones remains above 0.180 in natural weathering, while that of gas-bearing sandstones ranges from 0.13 to 0.17 under the same condition. It can be safely concluded, therefore, that the AVO analysis technique, referring to the the Passion's ratio, is practicable to predict the gas saturation in the Upper Paleozoic sandstone reservoirs. Recent years have seen its wide use in Yulin and Sulige areas, with a success ratio of gas-bearing detection over 80% in the exploration period. Geological information Logging information Seismic information Gas pool Engineering information Denoising Preliminary processing of the seismic target Preliminary processing of the target Preliminary processing of the logging target Frequency increase Match Caliper correction Time sampling Conventional qualitative seismic interpretation of reservoir thickness Geophysical analysis of the reservoir; Comprehensive calibration of the reservoir Automatic classification of seismic facies; Analysis of reflected wave-group features Seismic processing and interpretation of stratigraphy & lithology; qualitative interpretation of reservoir thickness Synthetic sonic logging constrained-inversion Logging technology Prediction of reservoir gas saturation Rarefaction pulse inversion Simulate annealing inversion AVO analysis Absorbing coefficient calculation Seismic multi-parameter inversion Pattern identification Chart 1 Flow chart of techniques for horizontal prediction of upper Paleozoic reservoirs
Petroleum science 2004 He-8 Type-I wave shape(Su-20 well) Type-l wave shape(Su-29 well) Type-llI wave shape( Su-26 well) He-8 with sandstone thickness He-8 with sandstone thickness He-& with sandstone thickness from 15 to 30n Fig. 4 Typical reflection model in the He-8 reservoir of the Sulige area He& m”以 L98636 Fig. 5 The seislog reverse impedance section on the L98636 line in the Sulige Temple investment and improving the economic benefits of 4.3 Effects of seismic exploration technology exploration Successful drilling exploration in Yulin and eismic data obtained in past three years have Sulige Temple areas shows that, for the subtle low- provided 85 well locations for gas exploration, among permeability reservoirs in Changqing, long well which 62 have seen industrious gas flows. Compared distance exploration by using techniques of seIsmIc of reservoir reservoir prediction on the basis of geological thickness prediction has increased from 75.3%to.6% system analysis and research, may help find and and the success rate of exploration wells from 67.9%to effectively control the distribution area of such 72.9% reservoirs, which is also a key to saving exploration o1994-2007ChinaAcademicJournalElectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
116 Petroleum Science 2004 Type- wave shape (Su-20 well) He-8 with sandstone thickness >30m Type- wave shape (Su-29 well) He-8 with sandstone thickness from 15 to 30m Type- wave shape (Su-26 well) He-8 with sandstone thickness <15m Fig. 4 Typical reflection model in the He-8 reservoir of the Sulige area Fig. 5 The seislog reverse impedance section on the L98636 line in the Sulige Temple area 4.3 Effects of seismic exploration technology Successful drilling exploration in Yulin and Sulige Temple areas shows that, for the subtle lowpermeability reservoirs in Changqing, long well distance exploration by using techniques of seismic reservoir prediction on the basis of geological system analysis and research, may help find and effectively control the distribution area of such reservoirs, which is also a key to saving exploration investment and improving the economic benefits of exploration. Seismic data obtained in past three years have provided 85 well locations for gas exploration, among which 62 have seen industrious gas flows. Compared with that before 1999, the accuracy of reservoir thickness prediction has increased from 75.3% to 82.6% and the success rate of exploration wells from 67.9% to 72.9%
VoL 1 No. 2 Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin 5. Logging gas layer identification and low- impedance gas layer, we effectively identified low-impedance gas layers by comprehensively such methods as array resistivity reversion, time-lapse As far as the tight low-permeability gas layer logging logging, and nuclear magnetic resonance pore structure identification and evaluation are concerned. the main sis. For example, in the He-8 member in the Su-20 methods adopted in our industry can be summed up in well from 3. 439. 4 to 3, 444.4m, the lowest resistivity is the following. The first is the diffusion of high-accuracy 169. m. the highest moveout reaches up to 288us/m, with igital logging and image logging. In all gas exploration good physical properties. But because of its very low well projects we used the CLS3700 instrument, which resistivity and nonnoticeable triangulation response, can map natural gamma ray spectrometer besides routine conventional methods would have been likely to interpret curves. For some wells, we adopted the world-advanced it as a water reservoir. The original layer resistivity Elips-5700 and Maxis-500 image logging series, to obtained through 2D resistivity reversion, is 23.52.m ensure the accuracy of data collection and the need of CMR shows that there are two peaks in the T, spectrum special research. The second is to ensure data accurac of this layer, with well-developed micro pores and by improving the diplex scaling of outdoor instruments generally high porosity, and so low impedance gas and indoor curve standardization. Thirdly, great reservoirs may be formed. On the FMI map, this layer is importance is attached to analyzing and accurately interpreting the logging response mechanism to deep-colored fleck, which shows that the solution pores are well developed. Nuclear magnetic resonance accurately describe the reservoir parameters by core scale interpretation shows a low saturation of free water, and drilling, thick-layer subdivision and classified evaluation. the comprehensive interpretation shows it is a gas Fourthly, on the basis of overall evaluation, we developed bearing member. A combined test of this member and an accurate logging interprettation model based on rock that between 3, 464.2 and 3, 469. 5m shows an open flow physical facies analysis, to draw a general picture from potential of 23 2581 x 10'm'ld(Fig. 6). Finally, focusing perspective of lithologic identification, diagenetic on comprehensive interpretation, we pay much attention rvoir facies analysis and classification of reservo not only to logging information, but also to first-hand ayers and improve the use of logging data. Fifthly, information about corin sample drilling time, gas according to the formation type of the upper Paleozoic logging and so on MI OKSPUP FMI HEQ DYNA 3443 Array lateral resistivity and 2-D reversion CMR nuclear magnetic T2 spectrum FMI resistivity image Fig 6 Log evaluation map of the He-8 low-impedance gas layer Su-20 well o1994-2007chinaAcademicjOurnalElectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
Vol.1 No.2 Complex Exploration Techniques for the Low-Permeability Lithologic Gas Pool in Ordos Basin 117 5. Logging gas layer identification and evaluation As far as the tight low-permeability gas layer logging identification and evaluation are concerned, the main methods adopted in our industry can be summed up in the following. The first is the diffusion of high-accuracy digital logging and image logging. In all gas exploration well projects we used the CLS3700 instrument, which can map natural gamma ray spectrometer besides routine curves. For some wells, we adopted the world-advanced Elips-5700 and Maxis-500 image logging series, to ensure the accuracy of data collection and the need of special research. The second is to ensure data accuracy by improving the diplex scaling of outdoor instruments and indoor curve standardization. Thirdly, great importance is attached to analyzing and accurately interpreting the logging response mechanism to accurately describe the reservoir parameters by core scale drilling, thick-layer subdivision and classified evaluation. Fourthly, on the basis of overall evaluation, we developed an accurate logging interprettation model based on rock physical facies analysis, to draw a general picture from the perspective of lithologic identification, diagenetic reservoir facies analysis and classification of reservoir layers and improve the use of logging data. Fifthly, according to the formation type of the upper Paleozoic low- impedance gas layer, we effectively identified some low-impedance gas layers by comprehensively using such methods as array resistivity reversion, time-lapse logging, and nuclear magnetic resonance pore structure analysis. For example, in the He-8 member in the Su-20 well from 3,439.4 to 3,444.4m, the lowest resistivity is 16Ω.m, the highest moveout reaches up to 288μs/m, with good physical properties. But because of its very low resistivity and nonnoticeable triangulation response, conventional methods would have been likely to interpret it as a water reservoir. The original layer resistivity, obtained through 2D resistivity reversion, is 23.5Ω.m. CMR shows that there are two peaks in the T2 spectrum of this layer, with well-developed micro pores and generally high porosity, and so low impedance gas reservoirs may be formed. On the FMI map, this layer is deep-colored fleck, which shows that the solution pores are well developed. Nuclear magnetic resonance interpretation shows a low saturation of free water, and the comprehensive interpretation shows it is a gasbearing member. A combined test of this member and that between 3,464.2 and 3,469.5m shows an open flow potential of 23.2581 × 104m 3 /d (Fig. 6). Finally, focusing on comprehensive interpretation, we pay much attention not only to logging information, but also to first-hand information about coring, sample log, drilling time, gas logging and so on. Array lateral resistivity and 2-D reversion CMR nuclear magnetic T2 spectrum FMI resistivity image Fig. 6 Log evaluation map of the He-8 low-impedance gas layer Su-20 well
l18 Petroleum science 2004 In the exploration of the Upper Paleozoic Yulin, basin underwent two sedimentary stages from the mid- Sulige and Shenmu gas pools, logging played an late Carboniferous offshore plain to the Permian important role in the timely and accurate discovery of the ental river and lake delta in the upper Paleozoic gas layer. The accuracy rates of logging etation The formations within are very well developed sand lave been kept over 85% for many years in the past bodies like delta plain river channels, delta front river mouth bars and tidal channels. These sand bodies. either 6. Low-permeability sandstone gas pool above or between source rocks fracturing sources, thus forming large areas litho Because the upper Paleozoic gas layers are low pressure and low abundance characterized by low permeability and low pressure, most With regard to the extreme difficulties in exploration of After many experiments, we developed a fracturing- the upper Paleozoic sandstone lithologic gas pools, a set of liquid system suitable for the low-pressure gas layers and focusing on geological appraisal, seismic exploration, construction techniques of CO2 energizing fracturing, accurate logging evaluation and interpretation, well testing large-volume sand fracturing, deep well liquid-nitrogen drainage, thus overcoming such technical difficulties as fracturing. These techniques provided an effective guide the return of drilling fluids in low-pressure-and- or the gas exploration in the upper Paleozoic and helped permeability gas layers and the pollution of the gas layer Mizhi. At the same time, they also provide methods and caused by fracturing liquids. After fracturing, the non initial or low-initial wells can produce industrious flows techniques available for exploration in other basins or become productive ones(Table 2) ontaining gas pools of the same type Table 2 Production-increase in the upper Paleozoic gas about the first author fracturing Fu Jinhua, male, born in 1962 Layer Perforated production production petroleum geology from Jianghan m)(m)(m7d) Petroleum Institute in 1983. He has beer engaged in petroleum exploration and Shan-9 Shanxi 2769-2777 150400 development of Ordos Basin for a long Shan-56 Shanxi 3714-3721 102883 Shan-I17 Shanxi 2914-2928 104438 period of time. Now he is the vice-president of Petroleum Shan-215 Shanxi2736~274640557 184144 Exploration Development Research Institute of the Yu-15Taiyuan 2152-21638164 116100 Changqing Oilfield Branch Company, PetroChina Company LimitedE-mail:xsl@petrochina.com.cn 7. Conclusions Against the special structural background, the Ordos (Edited by Yang Lei) 鄂尔多斯盆地上古生界低渗岩性气藏综合勘探技术 付金华席胜利刘新社孙六一 (中国石油长庆油田分公司勘探开发研究院,中国陕西西安710021) 摘要:鄂尔多斯盆地是一个稳定的克拉通,区内晚古生代经历了中晚石炭世近海平原至二叠纪内陆河 流、湖泊三角洲两个沉积阶段,沉积地层中三角洲平原河道、三角洲前缘河口砂坝、潮道砂体等各种 成因的砂体极为发育,砂体处于生烃源岩之上、或夹于生烃源岩之中,气源条件优越,形成大面积分 布的低孔、低渗、低压、低丰度隐蔽性砂岩岩性气藏。近几年,针对该岩性气藏的勘探难点,开展了 ˉ系列攻关实验,总结了一套以地质评价、地震勘探、测井精细评价解释、试井压裂等为主体的勘探 技术,有效地指导了上古生界天然气的勘探,发现了苏里格、榆林、乌审旗、米脂四个大气田。 关键词:鄂尔多斯盆地上古生界岩性气藏地震勘探测井精细评价勘探技术 o1994-2007ChinaAcademicJournalElectronicPublishingHouse.Allrightsreservedhttp://www.cnki.net
118 Petroleum Science 2004 In the exploration of the Upper Paleozoic Yulin, Sulige and Shenmu gas pools, logging played an important role in the timely and accurate discovery of the gas layer. The accuracy rates of logging interpretation have been kept over 85% for many years in the past. 6. Low-permeability sandstone gas pool fracturing Because the upper Paleozoic gas layers are characterized by low permeability and low pressure, most wells cannot produce industrial flows without fracturing. After many experiments, we developed a fracturingliquid system suitable for the low-pressure gas layers and construction techniques of CO2 energizing fracturing, large-volume sand fracturing, deep well liquid-nitrogen drainage, thus overcoming such technical difficulties as the return of drilling fluids in low-pressure-andpermeability gas layers and the pollution of the gas layer caused by fracturing liquids. After fracturing, the noninitial or low-initial wells can produce industrious flows or become productive ones (Table 2). Table 2 Production-increase in the upper Paleozoic gas layers through fracturing Well Layer Perforated layer (m) Initial production (m3 /d) Fractured production (m3 /d) Su-6 Su-5 Shan-9 Shan-56 Shan-117 Shan-215 Yu-15 Xiashihezi Xiashihezi Shanxi Shanxi Shanxi Shanxi Taiyuan 3318 3290 2769 3714 2914 2736 2152 ~3329 ~3314 ~2777 ~3721 ~2928 ~2746 ~2163 501420 90880 55965 2732 838 40557 8164 1201632 284699 150400 102883 104438 184144 116100 7. Conclusions Against the special structural background, the Ordos basin underwent two sedimentary stages from the midlate Carboniferous offshore plain to the Permian continental river and lake delta in the upper Paleozoic. The formations within are very well developed sand bodies like delta plain river channels, delta front river mouth bars and tidal channels. These sand bodies, either above or between source rocks, become good gas sources, thus forming large areas of subtle sandstone lithologic gas pools of low porosity, low permeability, low pressure and low abundance. With regard to the extreme difficulties in exploration of the upper Paleozoic sandstone lithologic gas pools, a set of lithologic gas pool exploration techniques is developed, focusing on geological appraisal, seismic exploration, accurate logging evaluation and interpretation, well testing fracturing. These techniques provided an effective guide for the gas exploration in the upper Paleozoic and helped discover four large gas fields, Sulige, Yulin, Wushenqi and Mizhi. At the same time, they also provide methods and techniques available for exploration in other basins containing gas pools of the same type. About the first author Fu Jinhua, male, born in 1962, received his bachelor's degree in petroleum geology from Jianghan Petroleum Institute in 1983. He has been engaged in petroleum exploration and development of Ordos Basin for a long period of time. Now he is the vice-president of Petroleum Exploration & Development Research Institute of the Changqing Oilfield Branch Company, PetroChina Company Limited. E-mail: xsl@petrochina.com.cn (Edited by Yang Lei)