四川盆地陆相层系天然气成藏条件与勘探思路

Gas accumulation conditions and exploration strategies of Mesozoic terrestrial strata in the Sichuan Basin

四川盆地油气勘探始于20世纪50年代,90年代初新场等气田的发现是一个划时代的标志,此后陆相层系勘探快速发展,呈现出全盆地砂岩气藏和页岩气藏兼顾的立体勘探格局。然而,该领域的天然气勘探开发也遇到了许多问题:储层多但大都较薄、相变快,储层物性整体较差且非均质性强,气藏分布广但资源丰度一般较低,勘探效果好、储量巨大但开发动用效果不理想等。为此,深入研究了四川盆地陆相层系天然气成藏条件。结果认为:①四川盆地陆相层系发育3套烃源岩(上三叠统须家河组须二段、须四段和侏罗系),烃源条件十分优越,烃源岩集中在川西、川东北和鄂西渝东等地区,初步计算四川盆地陆相层系生烃总量达550×1012 m3;②须家河组、侏罗系砂体发育,但在发育程度、物性特征等方面存在着横向差异,成岩作用是导致各地区储层类型及物性变化的主要因素之一;③四川盆地陆相气藏为生储广泛接触、大面积集群式成藏,各气藏间无统一的压力系统和气水界面,各层系各地区气藏成藏模式和控制因素均不同;④四川盆地陆相层系页岩气资源丰富,是下一步立体勘探的重要目标。进而提出了四川盆地陆相层系的勘探思路:应加大投入,砂岩油气与页岩气兼顾,以寻找和扩大油气生产阵地为目的,分"精细展开、甩开突破、超前准备"3个层次推进四川盆地油气勘探工作。最后提出了发展非常规油气勘探理论与技术、加强三维解释和气藏精细刻画、按非常规思路开展工作等建议。

Hydrocarbon exploration in the terrestrial strata started early in the 1950sin the Sichuan Basin and the discovery of gas fields like Hexinchang,Xinchang,etc.in the early 1990sbrought about an epoch-making influence.Along with the subsequent terrestrial exploration in fast speed,both sand gas and shale gas exploration are developing well in combination with the stereoscopic exploration in this basin.However,many difficulties have been encountered in actual exploration and development there.For example,there are multiple reservoirs but most of them are so thin and with such facies variation that they are characterized by poor petrophysical properties and strong heterogeneity;there are wide distribution of reservoirs but with a low resource abundance level;huge reserves is proven with satisfactory exploration results but are difficult to produce in reality.In view of this,this paper makes a deep investigation into the gas accumulation conditions in the terrestrial strata in this basin.The following results are presented.(1) There developed three sets of source rocks,i.e.the 2 nd and 4 th members of the Xujiahe Formation in the Upper Triassic and the Jurassic;the superior hydrocarbon sources are mainly located in the western and northeastern Sichuan Basin as well as the area between west Hubei and east Chongqing,where the total hydrocarbon quantity generated is estimated to be 550trillion m 3.(2) Sandbodies developed in the Xujiahe Formation and Jurassic strata,however,the distribution and physical characteristics of sandbodies varied horizontally,and diagenesis was one of the dominant factors for reservoir types and physical characteristics variations at different areas.(3) Terrestrial gas reservoirs in this basin has wide source-reservoir contacts and group accumulation in a large area,and they also have inconsistent pressure systems and different gas-water contacts,and the accumulation models and controlling factors vary in different formations and at different areas.(4) Rich shale gas resources have been found in the terrestrial strata there,so it is the next important target of further exploration in the future.On this basis,the strategies are also suggested for further hydrocarbon exploration in the terrestrial strata in this basin.(1) For searching and expanding gas producing areas,the exploration investment should be increased and both of sandstone gas and shale gas should be explored.(2) Great advance should be made in the gas exploration in the Sichuan Basin according to the following three steps : to make a delicate plan,to make breakthrough by all means,and to make full preparation in advance.(3) Theoretical and technical studies should be developed for unconventional hydrocarbon resource exploration with the help of 3Dseismic interpretation and fine description of reservoirs.