川西北地区茅口组海槽相地质特征及其油气地质意义

Geological characteristics of trough facies, Maokou Formation, northwestern Sichuan Basin: Implications for geology

川西北地区中二叠统茅口组钻井显示频繁,含气性好,但由于对茅口组相带认识不够深入,不了解储层分布特征,一直未获得具有经济开采价值的气藏。根据川西北剑阁—九龙山地区井下岩石的薄片观察及测井解释,认为区内茅口组顶部发育一套深水相有机质泥岩,为海槽相沉积,可与中下扬子地区的孤峰组对比。并以此建立茅口组孤峰段的测井识别标志,井震联合解释,刻画了川西北地区茅口组海槽相及环海槽台缘滩相展布特征。结论认为:(1)茅口末期已形成开江—梁平海槽雏形;(2)茅口组孤峰段岩性以薄层状深色有机质泥岩为主,呈北西—南东向展布,是潜在的优质烃源岩;(3)环海槽发育茅三段台缘滩相沉积,地震响应特征明显,台缘宽度约8~10 km;(4)川西北地区茅三段台缘滩相叠加岩溶作用易形成层状孔隙型储层,同时该套储层与孤峰段深水沉积侧向对接,具有良好的源储配置关系,是茅口组未来勘探的有利方向。

During drilling the Middle Permian Maokou Formation,northwestern Sichuan Basin,there were frequently shows occurring with better gas bearing property.But so far,no commercial gas reservoirs have been discovered.The main reasons are that the facies belt of Maokou Formation is not understood thoroughly and the reservoir distribution is not figured out.After thin-section observation and logging interpretation for Jian’ge-Jiulongshan area,northwestern Sichuan Basin,it is deemed that,at the top Maokou Formation,there is developed deepwater organic mudstones of trough facies,which may also be correlated with the Gufeng Member in the lower and middle Yangtze regions.An identification mark on Gufeng Member of Maokou Formation was established.And based on well-seismic interpretation,the distribution of both trough facies and marginal platform shoal facies around trough during Maokouian period in northwestern Sichuan Basin was summed up.Results show that(1)embryonic Kaijiang-Liangping trough had been formed by the end of Maokouian period;(2)Gufeng Member,which is dominantly composed of thin-laminated dark organic mudstones with NW-SE strike,is potential source rock with high quality;(3)for Maokou 3 Member with marginal platform shoal deposits developed around the trough,its seismic response characteristics are obvious,and this platform is about 8-10 km wide;and(4)under karstification,some layered porous reservoirs might form easily in the marginal-platform facies of Maokou 3 Member.In addition,this Maokou 3 Member,which laterally contacts the deepwater-facies Gufeng Member to form better source-reservoir assembly,should be the next favorable exploration target for Maokou Formation.