渤中凹陷深层砂砾岩气藏油气充注与储层致密化

渤海湾盆地渤中凹陷砂砾岩储层致密化成因及其与晚期油气侵位的关系一直没有得到合理地解释。为此,以渤中凹陷西南部地区古近系砂砾岩气藏为研究对象,利用物性、热史、包裹体等分析资料,确定油气首次成藏时间、划分油气充注期次、恢复成藏期储层物性,进而模拟压实作用对储层的影响,并结合包裹体、铸体、扫描电镜、X射线衍射等实验手段明确油气侵位关系,分析石英加大、黏土矿物转化、碳酸盐胶结等关键致密化作用发生的序列,探究储层致密化作用机理及其与油气充注的关系。研究结果表明:①该区深层砂砾岩油气充注至少可分为3期,早期包裹体为重质油,晚期包裹体气油比高;②首次油气充注期为距今5Ma,储层埋深介于2500～2800m,以中孔、中渗储层为主,储集物性较好,成藏后盆地快速沉降、充填,上覆地层增加厚度超过1 000 m;③首次油气充注后砂砾岩进入成岩快速演化期,埋深介于2 500～3 200 m,是砂砾岩快速压实阶段,石英加大在油气充注后经历了两期强烈发育,减孔作用明显,压实减孔作用是储层致密化的重要机制;④埋深介于2 500～3 500 m为该区黏土矿物快速转化区间,花状及丝发状伊利石极其发育,对渗透率的大小产生了决定性的影响;⑤铁方解石及铁白云石沉淀于石英加大之后,使储层进一步致密化,以残余孔隙为主。结论认为,该区古近系砂砾岩气藏具有先成藏后致密的特点,尽管油气充注对储层成岩具有抑制作用,但后者仍然会导致储层的致密化。

Hydrocarbon charging and reservoir densification of the deep-seated glutenite gas reservoirs in the Bozhong Sag

The glutenite reservoirs in the Bozhong Sag of the Bohai Bay Basin are characterized by dense lithology, poor reservoir physical property and late-stage hydrocarbon accumulation. And so far, no reasonable explanation has been reached on the relationship between the reservoir densification and the late hydrocarbon emplacement. In this paper, a Paleogene glutenite gas reservoir in the southwest of Bozhong Sag was taken as a research object. Its initial hydrocarbon accumulation time was determined, its hydrocarbon charging period was divided and its reservoir physical properties in the period of hydrocarbon accumulation were restored by using the analysis data of physical property, thermal history and inclusion. Then, the effect of compaction on the reservoir was simulated by means of compaction simulation test, and the relationship of hydrocarbon emplacement was figured out by conducting the experiments of inclusions, casting, scanning electron microscope (SEM) and X-ray diffraction. Finally, the sequence of key densification effects was analyzed, including quartz enlargement, clay mineral transformation and carbonate cementation, and the reservoir densification mechanism and its relationship with hydrocarbon charging were investigated. And the following research results were obtained. First, hydrocarbon charging in the deep-seated glutenite reservoir of the study area can be divided into 3 periods. Inclusion in the early period is heavy oil and that in the late period is of high gas/oil ratio. Second, the first hydrocarbon charging to date from 5 Ma, when the burial depth of the reservoir was in the range of 2 500-2 800 m, the reservoir of medium porosity and medium permeability was dominant and the reservoir physical properties were better. After hydrocarbon accumulation, the basin subsided and was filled quickly and its overlying strata are over 1 000 m thick. Third, after the first hydrocarbon charging, glutenite experienced a rapid diagenetic evolution at the burial depth of 2 500-3 200 m, which is classified as the rapid compaction stage of glutenite. After hydrocarbon charging, quartz experienced violent overgrowth of two phases and porosity reduction was obvious. Therefore, compaction and porosity reduction are the main mechanisms of reservoir densification. Fourth, at the burial depth of 2 500 m to 3 500 m, it is the rapid transformation zone of clay mineral in the study area and flower-like and the silk-like illite is quite developed, which has a decisive influence on the permeability. Fifth, ferrocalcite and ferrodolomite are precipitated after the quartz overgrowth and filled the residual pores, resulting in further reservoir densification. In conclusion, the Paleogene glutenite gas reservoir in the study area became dense after hydrocarbon accumulation, and diagenesis can still lead to reservoir densification even though it is suppressed by hydrocarbon charging.