BACKGROUND'High-quality layer' is the focus and hotspot of shale gas exploration and development. Many explorations and important developments have been studied by researchers at home and abroad. However, the threshold of selection parameters, qualitative-quantitative system classification evaluation and template establishment need to be optimized.

OBJECTIVESTo identify the 'high-quality layer' and optimize areas of marine shale gas with the commercial value in the Lower Paleozoic area of southern China.

METHODSThe Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and Cambrian Niutitang Formation were the research objects. The identification factors of shale gas enrichment and high-yield 'high quality layer', and the shale gas area with commercial exploration value in the Lower Paleozoic marine facies in southern China were analyzed.

RESULTSThree qualitative parameters including sedimentary facies, tectonic preservation, lithologic combination, and eight quantitative parameters including gas content, pressure coefficient, organic carbon content, organic matter maturity, porosity, brittleness, effective thickness, buried depth were optimized. A discriminating principle of qualitative parameters to delineate favorable areas, and quantitative parameters to identify the 'high-quality layer' was proposed. On the basis of this, the parameter acquisition methods and thresholds were analyzed. A 'high-quality layer' quantitative identification template for marine shale gas of the Wufeng-Longmaxi Formation and the Niutitang Formation was established in southern China. This method was applied to the Zheng'an-Wuchuan area of Guizhou Province. Seven favorable shale gas areas and one 'high-quality layer' of shale gas (the Wufeng-Longmaxi Formation of Anchang syncline) were identified, which successfully verified by later drilling.

CONCLUSIONSThe proposed method has certain reference significance for the identification of 'high-quality layer' of marine shale gas in southern China.