柴西湖相碳酸盐岩致密气藏地质特征及规模富集条件——以环英雄岭地区为例

Geological characteristics and enrichment conditions of lacustrine carbonate tight gas reservoir,western Qaidam Basin:Case study of Yingxiongling area

柴达木盆地西部(以下简称柴西)湖相碳酸盐岩地层分布广,天然气资源潜力大,勘探程度低,天然气富集条件不明。基于非常规理念,以环英雄岭地区已发现的古近系—新近系(E32-N21)碳酸盐岩气藏为例,综合气藏解剖、地球化学及实验分析等资料,重新认识和评价了气藏特征及其含气潜力。结果表明,储层以灰质云岩、泥灰岩为主,渗透率小于0.1×10^(-3)μm^(2)的储层占70%以上,孔隙类型主要为溶孔、晶间孔和裂缝,非均质性强,属于致密碳酸盐岩储层。气藏具有含气层系多、大面积分布、含气饱和度低、多藏聚集、产量变化大的特点,属于碳酸盐岩致密气藏。柴西发育E32和N1共2套优质气源,生气强度大,为大面积成藏提供了充足的动力;湖相碳酸盐岩平面上大面积分布,纵向多层叠置,多种成因的裂缝与溶孔、晶间孔组合形成的孔缝网络结构提供了优质的储集空间;源储叠置、交互发育的"三明治"结构为大面积成藏奠定了基础;先致密、后成藏的特点及稳定分布的大段泥、盐岩层为致密气的保存提供了良好条件。在以上条件分析基础上,指出柴西湖相碳酸盐岩致密气藏资源潜力大,是柴达木盆地天然气勘探重要接替领域,狮子沟—油砂山—英东、南翼山—小梁山和油泉子—油墩子构造带为有利勘探方向。

The lacustrine carbonate formations of the western Qaidam Basin are characterized by wide distribution,great potential of natural gas resources,low exploration degree and unclear gas enrichment conditions.Based on the unconventional theory,taking the discovered Paleogene-Neogene carbonate gas reservoir in Yingxiongling area as an example,the comprehensive analysis method of geological,geochemical and drilling data was used to recognize and evaluate the characteristics and gas-bearing potential of the gas reservoir.The results show that the reservoir is mainly composed of lime dolomite and marlstone,and the reservoir with permeability less than 0.1×10^(-3)μm^(2) accounts for more than 70%.The main pore types are dissolution pore,intergranular pore and fracture,with strong heterogeneity,belonging to tight carbonate reservoir.The gas reservoir has the characteristics of multiple gas-bearing strata,large distribution area,low gas saturation,multiple accumulation and large production variation.The western Qaidam Basin has two sets of high-quality gas sources,E32 and N1,with high gas generation intensity,which provides sufficient power for large-scale reservoir formation.Lacustrine carbonate is widely distributed horizontally and overlapped vertically,and the pore fracture network structure formed by the combination of fractures,dissolved pores and intergranular pores of various origins provides favorable reservoir space.The sandwich structure of source-reservoir overlapping and interbedding lays a foundation for large-scale reservoir formation.The characteristics of densification first and then reservoir formation and the stable distribution of thick mudstone and salt rock provide good conditions for the preservation of tight gas.Based on the analysis of the geological conditions,this paper points out that the lacustrine carbonate tight gas reservoir of the western Qaidam Basin has great resource potential and is an important replacement area for natural gas exploration in the basin.The Shizigou-Youshashan-Yingdong,Nanyishan-Xiaoliangshan and Youquanzi-Youdunzi structural belts are the next favorable exploration areas.