威远气田震旦系灯影组古岩溶与成藏探讨

An approach to the palaeokarsts and pool accumulation in the Dengying Formation (Sinian) of the Weiyuan gas field, Sichuan

四川威远震旦系灯影组为一套浅海相潮坪环境下沉积的藻白云岩沉积 ,其发育的古岩溶特征具有纵向上的多期性和横向上的不均性。根据古岩溶的特征 ,笔者将其划分为成层间岩溶 ,风化壳岩溶和缝洞系岩溶三种类型 ,并对其不同的判别标志进行了详细研究和论述。层间岩溶为碳酸盐岩地层在沉积期或成岩早期因抬升暴露地表 ,受到大气淡水渗入淋滤所发生的岩溶作用 ;风化壳岩溶系指碳酸盐岩地层长时间受到大气淡水的淋滤作用 ,由于淡水的渗入和循环 ,使地表被改造为岩溶型地貌 ,并伴随着风化壳形成而发育的岩溶 ;缝洞系岩溶是指碳酸盐岩地层进入埋藏期后 ,随着埋藏深度的增加 ,在地层中不断排出压释水作用下 ,碳酸盐岩中发生的岩溶。威远构造的灯影组的震四1顶的层间岩溶和震四2 顶的风化壳岩溶 ,成为威远气田气藏的主要储集层。尤其是震四2 顶风化壳岩溶发育时 ,形成一个古潜丘 ,孔洞发育的潜丘属于古圈闭类型 ,有利于油气的早期聚集成藏。

The Dengying Formation (Sinian) in Weiyuan, Sichuan is mostly built up of algal dolostones deposited in the shallow-marine tidal-flat environments, where the palaeokarsts are well developed. The palaeokarsts may be divided, according to their features, into three types: intraformational, weathering crust and porous palaeokarsts. The intraformational karsts resulted from the karstification produced by the leaching of the uplifted carbonate rocks during deposition and early diagenesis by meteoric water. The weathering crust karsts are associated with the surficial karst-type landform and weathering crust generated by the permenant penetrating and leaching of the carbonate rocks by meteoric water. When carbonate rocks are buried, the interstial water was steadily released with increasing depths, thus resulting in the formation of the porous palaeokarsts. The intraformational karsts and weathering crust karsts at the uppermost parts of Z4 1 and Z4 2, respectively are believed to be the most important reservoirs in the gas field. In particular, the weathering crust karsts developed as a porous buried hill at the uppermost part of Z4 2 may facilitate the early accumulation of oil and gas in this region.