The Genesis Of Laminar Carbonates From Jiyang Sub-Basin And Their Implicate For Shale Oil Accumulation

Shale oil has become a popular topic for global unconventional oil and gas research in the petroleum industry.In recent years,the attention of petroleum geologists has gradually turned to research on lacustrine shale oil.Some exploration efforts have indicated that laminar carbonates are closely related to the favorable depositional strata of shale oil.Because of the differences between conventional and unconventional oil,the lithofacies is a particularly remarkable controlling factor of shale oil accumulation.This research focused on inter-bedded layers of laminar carbonates as well as mudstone and shale in the Dongying depression of the Bohai Bay Basin in eastern China,and attempted to reveal the genesis and evolution of the carbonates by means of drill core observations,thin sections,high resolution scanning electron microscopic observations,and fluorometric,cathodoluminescence and geochemical analyses.Based on above analyses,this research divides different lithofacies(combinations);meanwhile,this research analyzes the petrology,X-ray diffraction(XRD),total organic content(TOC),shale oil-bearing(SOB)property and reservoir space of the study layers.All the above indicators can be used to analyze the relation between shale oil accumulation and different lithofacies(combinations).The results demonstrated that:(1)the carbonates could be categorized into two groups,crystalline carbonates and micritic carbonates;(2)the crystalline carbonates were well crystallized and greenish under fluorescent light;and(3)the micritic carbonates were not well crystallized and had biotic textures.It was discovered that the crystalline carbonates were mainly developed in the shale layer and were formed from the late-filling effects of the diagenetic cracks along the shale fissility planes.The micritic carbonates were formed from direct deposition controlled by biological effects in the stratified flows of the seasonal lake basins.There are five main lithofacies in the study layers,which are as follows: LF1: thin lenticular laminae of crystalline carbonates,LF2: laminae of micritic carbonates,LF3: thick massive lime mudstone,LF4: massive mudstone,LF5: black shale.There are two shale oil accumulation models;one model commonly appears in LF4 and LF5 and is called “self-generation and self-reservoir”;the other model commonly appears in LF1+5 and is called the crystalline carbonate reservoir.Thus,the favorable lithofacies(combinations)for shale oil accumulation are LF1+5,LF4 and LF5.In the petroleum industry,crystalline carbonates form “sweet spots”,which are most beneficial for shale oil accumulation and production.