Analysis And Exploration And Development Of Shaximiao Formation Sandstone Oil Reservoirs In The Sichuan Region Of The Jurassic Recommendations

Oil field in central Sichuan basin is one of the oldest oil field in China. Since 1954 it has been explored oil and gas for about 50 years in this area. Up to 2001, 1378 wells has been drilled, and more than 20 oil fields or structures which reserve oil have been found. Therefore oil production in central Sichuan basin contributed to Chinese petroleum industry. In the past few years, a great amount of oil has been produced from the sandstone reservoir of Lianggaoshan formation and lower Shaximiao formation in Lianchi area and Gongshanmiao area. Up to now, accumulation proven oil reserve in Gongshamiao area has been risen to 5000 ton, and almost all oil reserved in the depth less than 3000 meter. Thus central Sichuan basin is a good prospective area for oil exploration.With studying well core, source rock, geochemistry of original oil, data of analyzing reservoir samples, seismic inversion, we conclude the rule and model of forming the oil reservoir for clastic rock in Lianggaoshan and lower Shaximiao formation. And on the bases of all studied results, the significant exploration policy is provided. The major conclusion is as following.1. With much more working on regional stratigraphic correlation, sedimentology, current orientation of ancient river, paleogeograthy evolution is rebuilt from Ziliujing to upper Shaximiao in upper-middle Jurassic. For example, it is considered that in Shaximiao period the major sedimentary facies developed in the study area are fluvial plain, delta front and shallow lake, which is quite different from the previous view of flood basin. Moreover, it is the first time that we point out one of the deposit sources extend from north to south and another one is from east to west.2. In the study area, the source rock formed in semi-lake of Daanzhai and Lianggaoshan period is distributed in central Sichuan basin. Residual Organic carbon in the source rock is abundant, and the kerogen types are mainly sapropel-type or humosapropelic-type. The hydrocarbon kitchen is mainly located in Yilong-Pingchang area, where vitrinite reflectance of organic matter is about 1.7-1.8%. With comparing gonane between oil and source rock, it is concludedthat oil reserved in the central Sichuan basin does not come from local place, but come from Yilong-Pingchang area.3. With studying core, outcrop and thin sections, laumontite cement is firstly found in the Shaximiao sandstone. Moreover the secondary pore space generated by dissolving laumontite cement is the most significant porosity in the sandstone reservoir. Also, statistics data from about 800 porosity and permeability indicate that reservoir quality in lower Shaximiao sandstone is much higher than in upper Lianggaoshan formation.4. For the specific sandstone reservoir in central Sichuan basin, seismic prediction techniques have been applied in predicting sheet sandstone of lacustrine facies in Lianggaoshan formation, thin sandstone of river channel in lower Shaximiao formation, think sandstone of river channel in lower Shaximiao formation. And the prediction results are more effective.5. In central Sichuan basin, most of Jurassic oil reservoir belong to lithologic-joint and fault-lithologic reservoir. With integrated study, distribution of source rock, ancient uplift, fault or joint, and distribution of sandstone reservoir are the essential key factors to form oil pool. Moreover, the faults which connects the source rock to sandstone reservoir is the most important one.6. To sum up all petroleum geological factors in central Sichuan basin, it is considered that oil exploration is prospective, although there are many problems to deal with. It is predicted that Gongshanmiao area, Nanchong-guangan-longniushi old oil field, Shuanghechang which is located in the slope of north area of central Sichuan basin, and Tianchi area are all major targets for future oil exploration.7.Because reservoir of Shaximiao formation has character of lower porosity and permeability and potential possibility of severe formation pollution, it is suggested that we should pay more attention to reservoir pollution by drilling mud, reservoir auto-absorbing water, stress sensitivity and additive.8. Because fracture is the important oil migration path for Lower Shaximiao member, it is suggested that horizontal well technique will be used to developing oil, in order to consider reservoir pressure, sensitivity, distribution of formationstress around wells. At same time, for the specific character of the formation it is suggested that low-pressure drilling technique should be used with premium operating fluid, such as gas drilling, mist drilling, foam drilling, aerated mud drilling, under balance pressure drilling and light mud drilling.