Tectonic-sedimentary differentiation and hydrocarbon source rock-reservoir distribution pattern of the Ordovician sub-salt sequence in the Ordos Basin

The carbonate-evaporite system in the sub-salt sequence has been a key area for increasing oil and gas reserves and production. Its source rock-reservoir combination styles and distribution have become the key constraints on oil and gas exploration. Analyzing the development and distribution pattern of hydrocarbon source rock-reservoir in the sub-salt carbonate-evaporite system in the Ordos Basin， we explore the controlling factors. The results show that： the hierarchical “uplift-depression” structural pattern determines the sedimentary substrate variability， and the dry and wet paleoclimate changes alter the oceanic conditions， leading to sedimentary differentiation， which results in the formation of different carbonate-evaporite depositional sequences， and the development of various hydrocarbon source rocks and reservoirs. Paleo-uplifts and subordinate uplifts control the distribution of hydrocarbon source rocks for arid-climate argillaceous dolomites， and the paleo-depressions are favorable for the growth of hydrocarbon source rocks for wet-climate micrites； grainstone deposits are prone to developing in paleo-uplift and subordinate uplift zones， which are also a favourable habitat for benthic organisms. These zones are the dominant zones for the reflux of high-salinity brine in the process of the climate shifting from wet to arid， where the occurrence of dolomitization is widely seen， controlling the distribution of crystalline dolostone， grain dolostone， microbial dolostones and porphyritic dolostone reservoirs. Tectonic-sedimentary differentiation governs the distribution pattern of hydrocarbon source rocks and reservoirs in the carbonate-evaporite system. The slope zone around the Central paleo-uplift and the Wushenqi uplift and Shenmu-Mizhi low uplift zone adjacent to the paleo-depression have effective hydrocarbon source rock- reservoir combinations developed， favourable zones for oil and gas exploration in the sub-salt sequence.