四川盆地达州—开江古隆起沉积构造演化及油气成藏条件分析

四川盆地乐山—龙女寺古隆起震旦系—下古生界天然气勘探所取得的重大突破,使得大型古隆起是该盆地深层天然气勘探重要目标的认识成为共识。然而,除了乐山—龙女寺古隆起之外,该盆地其他大型古隆起均被认为是后加里东期的古隆起,后者对震旦系—下古生界油气成藏的影响则极少被论及。为此,从分析野外露头、钻井及地球物理等资料入手,剖析了川东地区震旦系—下古生界沉积构造背景,研究了古地貌控制下的储层、烃源岩分布规律,结合构造演化分析成果,探讨了该区震旦系—下古生界含油气地质条件。结果表明:(1)中寒武世前川东北部发育达州—开江继承性隆起,控制震旦系、寒武系滩相储层大面积发育;(2)古隆起高部位发育滩相储层,其周缘凹陷带发育上震旦统陡山沱组、下寒武统笻竹寺组厚层烃源岩,源储配置条件优越;(3)二叠纪以来,达州—开江古隆起区由斜坡带逐步抬升,二叠纪生油高峰期处于斜坡—低缓隆起带,三叠纪生气高峰期处于隆起高带,有利于油气富集成藏;(4)川东地区下构造层(下寒武统及其以下地层)现今具备大型隆起构造背景,构造形态简单,其上完整的局部构造圈闭发育,保存条件好。结论认为,川东地区达州—开江古隆起区震旦系—下古生界具备良好的油气成藏条件,该古隆起高部位滩相储层发育区面积达8 000 km~2,是四川盆地下一步天然气勘探新的、重要的领域。

Sedimentary tectonic evolution and reservoir-forming conditions of the Dazhou–Kaijiang paleo-uplift,Sichuan Basin

Great breakthrough recently achieved in the Sinian–Lower Paleozoic gas exploration in the Leshan–Longnüsi paleo-uplift, Sichuan Basin, has also made a common view reached, i.e., large-scale paleo-uplifts will be the most potential gas exploration target in the deep strata of this basin. Apart from the above-mentioned one, the other huge paleo-uplifts are all considered to be the ones formed in the post-Caledonian period, the impact of which, however, has rarely ever been discussed on the Sinian–Lower Paleozoic oil and gas reservoir formation. In view of this, based on outcrops, drilling and geophysical data, we analyzed the Sinian–Lower Paleozoic tectonic setting and sedimentary background in the East Sichuan Basin, studied the distribution rules of reservoirs and source rocks under the control of paleo- uplifts, and finally discussed, on the basis of structural evolution analysis, the conditions for the formation of Sinian–Lower Paleozoic gas reservoirs in this study area. The following findings were achieved. (1) The Dazhou–Kaijiang inherited uplift in NE Sichuan Basin which was developed before the Middle Cambrian controlled a large area of Sinian and Cambrian beach-facies development. (2) Beach-facies reservoirs were developed in the upper part of the paleo-uplift, while in the peripheral depression belts thick source rocks were developed like the Upper Sinian Doushantuo Fm and Lower Cambrian Qiongzhusi Fm, so there is a good source–reservoir assemblage. (3) Since the Permian epoch, the Dazhou–Kaijiang paleo-uplift had gradually become elevated from the slope zone, where the Permian oil generation peak occurred in the slope or lower and gentle uplift belts, while the Triassic gas generation peak occurred in the higher uplift belts, both with a favorable condition for hydrocarbon accumulation. (4) The lower structural layers, including the Lower Cambrian and its underlying strata, in the East Sichuan Basin, are now equipped with a large-scale uplift with a simple structural configuration, above which a complete local structural trap was well developed with a good preserving condition. In conclusion, there are good forming conditions for hydrocarbon reservoirs in the Sinian–Lower Paleozoic strata in the Dazhou–Kaijiang paleo-uplift, East Sichuan Basin, in the higher parts of which the well-developed beach-facies reservoirs occupy an area of 8 000 km² and will be the next important gas exploration target in this basin.