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Application of Optical Microscope and SEM to the Organic Matter Identification in the Permian Lucaogou Formation of the Jimsar Sag, Junggar Basin, NW China: An Implication for the Depositional Environment

Abstract

The Lucaogou Formation is located in the Jimsar Sag, Junggar Basin, northwest China. The formation has a great potential for tight oil resources, which is analyzed here in detail. We applied optical microscopy and scanning electron microscopy (SEM) to a set of mudstone samples recovered from the Lucaogou Formation (mainly from “sweet spots”), with the aim to identify the origin of the organic matter, and to evaluate the depositional environments. Clearly this source rock has a good hydrocarbon generation potential, as evidenced by a high total organic carbon content (TOC 0.16-13.86 wt%) and a high hydrogen index (HI 10.5-1097.9 mg HC/g TOC). Maximum temperature for hydrocarbon volatilization (Tmax 434-459℃) suggests that the Lucaogou Formation is at the early mature to mature stage. Amorphous organic matter (AOM) was locally-flocculent and surrounded by minerals, stripe-like parallel or sub-parallel to the bedding, or finely-dispersed mingling with minerals via the optical microscope. In the high-resolution SEM images, continuous AOM was disturbed locally by structured organic material (e.g., algainite, inertinite). The fluorescence of AOM was controlled by its origin and abundance, which is directly related to the depositional conditions. Poor fluorescent or non-fluorescent vitrinite and inertinite were the predominant macerals in the source rock with type III kerogen. In the Lower Lucaogou Formation organic matter was dominated by phytoclasts and AOM; their ratio increased with depth, indicating that a relatively shallow, moderate-energy depositional environment was followed by a deeper, low-energy condition with rising lake level. Redox conditions changed from proximal suboxic/dysoxic-anoxic to distal suboxic-anoxic, as suggested by the ternary APP plot. Whereas in the Upper Lucaogou Formation, organic matter dominated by AOM (with limited terrestrial influx) precipitated in a shallow, stratified, evaporative lacustrine depositional environment, which is consistent with the distal suboxic-anoxic redox condition inferred from the ternary APP plot.