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.net112 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 non￾geological 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