玛湖凹陷西斜坡区三叠系白碱滩组沉积特征

Sedimentary Characteristics of the Triassic Baijiantan Formation in the Western Slope Area of the Mahu Depression

最近在玛湖凹陷斜坡区上三叠统白碱滩组砂体内钻遇工业油流,但其沉积成因还未见报道。以玛西斜坡区为研究范围,在岩心沉积特征观察、镜下岩矿成分分析及粒度分析等基础上,结合测、钻井资料等,对白碱滩组沉积相类型进行鉴定,并阐明沉积演化规律。结果表明:白碱滩组内包含辫状河三角洲和湖泊两个相类型,并可进一步细分出辫状河三角洲前缘、前辫状河三角洲及浅湖共计3个亚相类型,以及水下分流河道、河口砂坝、远砂坝、浅湖砂坝等7个微相类型;纵向上,自白一段到白三段,表现为自浅湖亚相到辫状河三角洲前缘亚相的连续过程,演化规律十分清楚。白碱滩组三段内发育的大面积优质水下分流河道储集砂体,与三叠系—侏罗系区域性不整合面、玛湖凹陷斜坡区中浅层的正断层等组合较好,可形成良好的圈闭与油气成藏条件,应是今后玛湖凹陷西斜坡区白碱滩组油气勘探的首选目标。该研究对于玛湖凹陷斜坡区三叠系白碱滩组今后的油气勘探工作具有重要的理论指导意义。

Hydrocarbon resources have previously been discovered in the Triassic Baijiantan Formation sandstone in the slope area of the Mahu Depression,but the depositional genesis has not been reported to date. The present study used core pattern observation,rock composition analysis and grain-size analysis combined with logging and drilling data,among other techniques,to determine the sedimentary facies type and evolutionary processes of the Baijiantan Formation. The development of a braided river delta and lake facies was established,which comprise a braided river delta front,and pro-delta and shallow-lake subfacies. Seven microfacies were recognized in the subaqueous distributary braided channel,mouth bar,distal bar and shallow lake bar. A continuous progradational process from shallow lake to braided river delta front was very clearly observed extending from the first to the third member. Ideal trap and hydrocarbon reservoirs conditions have been developed by favorable high-quality subaqueous distributary braided channel sandstone over the greater part of the third member,with a Triassic–Jurassic unconformity and overlying Jurassic mudstone and normal faults. It is recommended that this area should be the primary target of hydrocarbon exploration on the western slope area of the Mahu Depression. This study also has significant theoretical implications for subsequent hydrocarbon exploration in the area.