四川盆地自贡地区中二叠统茅口组多类型储层特征及分布预测

Characteristics and prediction of multi-type reservoirs in the Middle Permian Maokou Formation in Zigong area,Sichuan Basin

为了厘清四川盆地自贡地区中二叠统茅口组储层的差异发育机制与分布规律,基于丰富的测井、录井、取心和测试资料,对储层类型及主控因素进行了综合分析。研究结果表明:区内主要存在石灰岩类、白云岩类两种储集岩类。白云岩类储层主要发育于茅二段和茅四段,一般单层厚十几至几十厘米,多发育于向上变浅序列顶部,平面上茅二段白云岩储层主要分布于威远—自贡—富顺一线,而茅四段白云岩储层则主要分布于观音场—青杠坪一带;石灰岩类储层厚度变化大、非均质性强,纵向上可发育于茅二段—茅四段,平面上主要分布于沉积期地貌或风化壳岩溶地貌的高地或斜坡。进一步分析表明,白云岩储层形成与高频海平面下降驱动的蒸发浓缩—回流渗透白云石化和短期暴露岩溶相关,其中沉积期地貌相对坡折和滩体叠置迁移加剧了研究区局限蒸发的海水环境;石灰岩储层主要有滩控早成岩期岩溶、风化壳岩溶和断溶优化3种成因类型,分别受沉积期地貌坡折、岩溶地貌高地和斜坡以及断裂控制。综合分析认为,区内茅口组强非均质性、多成因类型储层具有受沉积期古地貌、古环境、风化壳岩溶古地貌、古断裂等“四古”复合控储特征。有利储层具有“沿高环坡”的分布规律,下一步勘探方向应重视东北部自贡—富顺区带的多层系、多成因类型储层的立体勘探和西南部观音场—青杠坪一带规模性白云岩储层的勘探。

In order to identify the differential development mechanism and distribution law of reservoirs in the Middle Permian Maokou Formation in Zigong area in Sichuan Basin,reservoir types and main controlling factors are comprehensively analyzed by using abundant wireline logging,mud logging,core samples and well testing data.The study results show that there are two main reservoir types in the study area,i.e.,limestone and dolomite.The dolomite reservoirs were mainly developed in the second member(Mao 2)and the fourth member(Mao 4)of Maokou Formation,with a single layer thickness of over ten to tens of centimeters,and generally developed at the top of upward shallowing sedimentary sequence.Laterally,the dolomite reservoir in Mao 2 member is mainly distributed in Weiyuan-Zigong-Fushun area and that in Mao 4 member mainly in Guanyinchang-Qinggangping area.The limestone reservoirs show a large variation in thickness and strong heterogeneity,which are distributed in Mao 2-Mao 4 members vertically,and the paleo highland or slope zone of the depositional or weathering crust karst geomorphology laterally.The further analysis shows that the formation of dolomite reservoirs was related to evaporation concentration-reflux infiltration dolomitization and short-term karst exposure driven by high-frequency sea level drop,and the limited evaporative seawater environment was intensified in the study area by the relative slope break during the depositional period and overlapping migration of beach bodies;The genesis of limestone reservoirs was classified into types,i.e.,beach controlled early diagenetic karst,weathering crust karst and fault dissolution reconstruction,which were controlled by slope break,highland or slope of karst landform and fault,respectively.It is concluded that the highly heterogeneous and multi-genetic reservoirs in Maokou Formation were jointly controlled by“four-paleo”factors,namely,paleo geomorphology during the depositional period,paleo environment,paleo weathering crust karst geomorphology,and paleo faults.The favorable reservoirs are distributed along the“circum-slope highland”.The exploration orientation in the near future includes the stereoscopic exploration of multi-layer and multi-type reservoirs in Zigong-Fushun area in the northeast and the large-scale dolomite reservoirs in Guanyinchang-Qinggangping area in the southwest.