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.net2004 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 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 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 low￾permeability 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