深层海相页岩气立体开发“甜点”岩相及其成因机制——以四川盆地南部泸州区块龙马溪组一段一亚段为例

Sweet spot lithofacies and its genesis mechanism for stereoscopic development of deep marine shale: a case study of the first submember of Member 1 of Longmaxi Formation in Luzhou area, southern Sichuan Basin

页岩岩相控制着页岩的生烃潜力、储集性能以及可压裂性。基于岩心观察结果,综合利用岩石薄片鉴定、X射线衍射(XRD)矿物分析、场发射扫描电镜(FE-SEM)成像、X射线荧光光谱(XRF)元素扫描、主量/微量元素测试等技术手段,对四川盆地南部泸州区块龙马溪组一段(龙一段)一亚段上部开发层系(上层系)页岩岩相进行了系统研究。研究结果表明,龙一段一亚段上层系主要发育高TOC含泥硅质页岩(S1)岩相、高TOC硅泥混合页岩(CM1)岩相、中TOC含泥硅质页岩(S2)岩相、中TOC硅泥混合页岩(CM2)岩相、中TOC混合页岩(M2)岩相和低TOC硅泥混合页岩(CM3)岩相6种岩相。其中,S1岩相和CM1岩相为龙一段一亚段上层系“甜点”岩相类型。S1岩相主要发育于龙一段一亚段4小层底部和龙一段一亚段6小层下部,有机质丰度高,有机孔及微裂缝发育,脆性矿物含量高,现场测试其含气量最高,为最优岩相类型;CM1岩相主要发育于龙一段一亚段6小层中—下部,有机质丰度高,黏土矿物晶间孔、有机孔及微裂缝发育,现场测试其含气量较高,为次优岩相类型。海平面升降、火山事件以及上升洋流共同控制着泸州区块龙一段一亚段上层系“甜点”岩相的形成。Ce元素含量异常表明,海平面小幅上升造成底层水中溶解的氧浓度降低,同时火山喷发的火山灰释放大量营养元素,以及上升洋流带来的深海富营养水体,共同促使表层海水中海洋生物生产力提高,形成了一套富有机质的硅质、硅泥混合页岩,并在成岩过程中发育了大量微孔隙和微裂缝。

Lithofacies control the hydrocarbon generation potential, reservoir property and fracturability of shales. Based on core observations, comprehensively using the techniques such as rock thin sections identification, X-ray diffraction mineral analysis, field emission scanning electron microscopy(FE-SEM) imaging, X-ray fluorescence(XRF) element scanning and major element and microelement testing, the study systematically investigates the shale lithofacies of the upper layer series of development in the first submember of Member 1 of Longmaxi Formation(Long1 Member) in Luzhou area of Sichuan Basin. The results show that the study area mainly develops six lithofacies, i.e., high TOC siliceous shale(S1), high TOC siliceous clay mixed shale(CM1), medium TOC siliceous mud shale(S2), medium TOC siliceous clay mixed shale(CM2), medium TOC mixed shale(M2) and low TOC siliceous clay mixed shale(CM3). Among them, S1 and CM1 are the sweet spot lithofacies of the upper layers in the first submember of Long1 Member. S1 lithofacies is mainly developed at the bottom of the fourth layer and the lower part of the sixth layer in the first submember of Long1 Member, characterized with high organic matter, developed organic matter pores and microcracks, high content of brittle minerals, and the highest gas content, which is classified as the optimal lithofacies type. CM1 lithofacies is mainly developed in the middle and lower parts of the sixth layer in the first submember of Long1 Member, characterized with high organic matter, well-developed intergranular pores of clay minerals, organic matter pores and microcracks, as well as high gas content, which is classified as the suboptimal lithofacies type. The formation of sweet spot lithofacies is jointly controlled by sea level changes, volcanism and upwelling ocean currents. Ce anomalies indicate that a slight rise in sea level causes a decrease in dissolved oxygen concentration in the bottom water. Meanwhile, volcanic ashes release a lot of nutrient elements, and the upwelling ocean currents can provide eutrophic water bodies, thus promoting the marine biological productivity in surface sea water, and forming a set of organic-rich siliceous shale and siliceous clay mixed shale. Moreover, a large number of micro pores and micro cracks were developed during the diagenesis process of the shales.