四川元坝地区须家河组沥青发育分布特征及其烃源岩排烃通道标志

Distribution and Origin of the Bitumen and Its Trace for the Hydrocarbon-expelling Pathway of Xujiahe Formation in Yuanba Area of Sichuan,China

四川元坝地区须家河组为一套含煤层系。基于岩芯观测、沥青有机地球化学分析和沥青产状描述,发现泥质烃源岩与砂砾岩储层裂缝中广泛发育固体沥青,这些固体沥青是黏稠状原油裂解的产物;应用三芳甾烷分子标志物对比证实,该原油是由煤系泥质烃源岩中藻类为主的有机质生成的,从烃源岩排出过程中因黏稠度高而残留于运移通道中,成为排烃通道的识别标志。结合须家河组超压发育演化,建立了该层系超压驱动下烃源岩的排烃通道模式,烃源岩排烃可以分为时空上连续的两个过程,即油气初始凝聚于微裂缝和其后经(微)裂缝的排放。初始凝聚过程中烃类的相态、通道与排驱动力学还知之甚少,(微)裂缝排放通道主要由顺层理面及穿层的水力破裂缝组成,强超压与构造应力是(微)裂缝排放通道形成的动力学机制,由此推断超压水力破裂缝与构造裂缝是烃源岩排烃的主要通道。

Xujiahe Formation in Yuanba area of Sichuan,China,is composed of coal-bearing strata.Based on core observation and measurement,as well as organic geochemical analysis and bitumen-occurrence description,it is demonstrated that widespread solid bitumen presents within the fractures among argillaceous source rock and sandstone-conglomerate reservoir layers.The precursor of bitumen（namely palaeo-viscous oil）is generated from preferentially algae-richorganic matter in argillaceous source rocks,proved by using relative abundance correlation of triaromatic steroids contained in solid bitumen and extracted from argillaceous source rock and coal rock.Being viscous,the palaeo-oil easily remains along expelling route,consequently tracing the oil-expelling pathway.Combined with the analysis of overpressure evolution for Xujiahe Formation,a conceptual model of oil-expelling pathway is created,namely,a representative oilexpelling pathway model driven preferentially by overpressure.Oil-expelling pathway consists of two tempo-spatially continuous processes,including initial sweating and agglomerating into micro-fractures,and subsequent discharging along the micro/macro-fractures.Little is known about the initial process;but authentically,the discharging or expelling fractures are composed of hydraulic fractures occurred along laminae and bedding as well as across both of them,mainly formed by strong overpressure and tectonic stress.Consequently,the hydraulic fracture and the tectonic micro/macro-fracture are the most important expulsion-conduits.