The genesis and evolution of laminar carbonates in lacustrine basin: implications for shale oil accumulation

Shale oil has become a popular topic for global, unconventional oil and gas research in the petroleum, industry. Recent exploration efforts have indicated that, laminar carbonates are closely related to the favorable, depositional strata of lacustrine shale oil. 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, petrological analyses. The results demonstrated that: the, carbonates could be categorized into two groups,, crystalline carbonates and micritic carbonates; the, crystalline carbonates were well crystallized and greenish, under fluorescent light; and 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, 'selfgeneration and self-reservoir; the other model, commonly appears in LF1 and is called the crystalline, carbonate reservoir. Thus, the favorable lithofacies, (assemblages) for shale oil accumulation are LF1, LF4 and, LF5. In the petroleum industry, crystalline carbonates, form 'sweet spots', which are most beneficial for shale oil, accumulation and production.