Scanning electron microscopy (SEM) images were acquired from core samples before and after low-temperature hydrous pyrolysis (LTHP) representing a variety of rock types from productive tight oil reservoirs of the Wolfcamp shale in the Delaware Basin, Texas. All of the samples yielded moderate-gravity oil (33°–41° API) via LTHP, regardless of lithology and total organic carbon (TOC) content, with the greatest amount of oil released from a tight sandstone sample recovered from the cored Wolfcamp X-Y sands interval. Retained bitumen, identified by its scaly appearing surface texture in SEM images, was observed in the post-LTHP samples. Interpreted oil-prone organic matter appeared unaltered between the as-received and post-LTHP samples, indicating no oil was generated from the thermal degradation of kerogen during LTHP. As-received mineral and organic matter pores also appeared unaltered following LTHP. However, dolomite moldic pores and organo-moldic pores were observed only in the post-LTHP samples, providing evidence of carbonate dissolution and hydrocarbon mobilization during LTHP. Oils released from the organic-lean sandstone and siltstone samples from the cored X-Y sands interval are interpreted as oils migrated from intraformational source rocks, whereas oils released from the higher TOC carbonaceous mudstone samples are interpreted as in situ generated oils retained in the thermally mature source rock facies. Variations in API oil gravity between the migrated and in situ oils are most likely the result of variation in volatile hydrocarbon loss during core retrieval and storage based on gas chromatography of the released oils, rather than reflecting oil generated from different source rocks.

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