湘中坳陷晚古生界储层成岩演化与天然气成藏

湘中坳陷是晚古生代南华陆表海盆在湘中的一个负向构造单元, 先天成油气地质条件优厚, 目前发生气喷井18口, 岩心检测储层物性整体表现为低孔低渗, 属于物性较差的致密储层。本文通过铸体薄片、电镜扫描、粒度图像、粒度孔隙等资料开展成岩组构镜下详细观察及孔隙演化特征分析。结果表明: 该区储层类型主要是孔隙型、裂缝型和孔隙-裂隙型, 其中次生溶蚀孔和后期构造产生的破裂缝为主要储集空间。下石炭统致密储层在距今约290 Ma时形成, 成岩作用对储层孔隙影响较大, 压实作用是使碎屑岩孔隙变差的主要控制因素, 压溶作用、溶蚀作用、破裂作用对储层物性有所改善, 后期的胶结作用使储层致密性加强。结合流体包裹体测温资料及区域地质背景与地层埋藏史、油气充注期, 指出该区天然气藏为先致密后成藏的天然气成藏模式, 深盆气为勘探主要潜力目标。

Diagenetic evolution of reservoirs and reservoir-forming stages in the Xiangzhong depression

The Xiangzhong depression is one of the negative structural unit of Nanhua land-surface sea basin in Late Paleozoic and possess favourable conditions for oil-gas generation. There are 18 gas production well at present, while the overall reservoir properties of the core detection shown as low porosity and permeability, belongs to poor physical property, tight reservoirs. The observation of diagenetic fabric and pores evolution were analyzed based on casting thin sections, SEM, rock grain size, granularity and porosity. The results show that: Porous type, fracture type and porous-fracture type are the main pore types in this area; Secondary denudation pore and tectonic fractures are the main reservoir spaces. Tight reservoirs in Lower Carboniferous were formed since 290 Ma. Diagenesis is impact a lot on reservoir porosity. As compaction is the main factor caused the loss of porosity; Pressure solution, corrosion, fracture change the pore structure effectively on sandstone reservoirs; Later cementation enhanced the tight of reservoirs. Combined with liquid inclusion homogenous temperature research, regional geological background, stratigraphic burial history and the infusing time, we concluded that before the gas accumulated in the trap, the reservoirs compacted. So the precedent deep basin gas is the main potential exploration targets.