Source And Accumulation Modes Of Natural Gas In The Carboniferous-permian In Southern North China Basin

The exploration and development of shale gas in the South China North Basin is in the initial stage,with lithologies of the coal-bearing formations in the Carboniferous-Permian transitional stages changing fast and the different lithology interlayers changing frequently.This paper aims at analyzing the geological background,the geochemical characteristics of source rocks,the characteristics of natural gas geochemistry,and the origin and source of natural gas of the Carboniferous-Permian coal-bearing formations in the Southern North China Basin.The reasons for the carbon isotopic reversal of the over-mature shales were discussed and the basic models of the natural gas accumulation in the transitional phase were established.The development of the Carboniferous-Permian source rocks in the South China North Basin is mainly affected by the sedimentary environment.There are large deposition thicknesses in the northwest of the basin decreasing along the southeast direction.The Shanxi Formation is delta deposit,whereas the Taiyuan Formation is shallow water shelf deposit.The organic matters are type III,and the overall TOC contents of the Taiyuan Formation are higher than that of the Shanxi Formation,which are generally at the high thermal stages.Natural gas in the South China North Basin is typical dry gas with relatively high dry coefficients.Gas desorption and isotope analyses reveal that CO₂ is organic thermogenesis and N₂ is hybrid origin of the high temperature metamorphic release of organic matter and clay minerals.Different lithologies have different isotope distribution fields.There is certain regularity according to lithologic changes.That is,natural gas is transported from shales and coals to sandstones and limestones.The carbon isotopic reversal of alkane gas is mainly caused by Rayleigh fractionation under high-temperature conditions at high mature and over mature stages.The samples collected from the well Zhengxiye-1 are thermally mature,and the carbon isotopic reversal recovers in a positive sequence.Source rock thermal simulation experiment and the isotherm adsorption experiment indicate that the Shanxi and Taiyuan Formations in South China North Basin have good hydrocarbon generation and accumulation conditions.Gas accumulation models can be divided into coal source,shale source,and the combination of coal and shale sources.Besides,it is concluded that the Taiyuan Formation with calcareous rock acting as both the top and bottom is a favorable model for natural gas accumulation.This paper systematically studied the geochemical origins and accumulation patterns of natural gas in transitional shales,and results will have certain values for disclosing gas accumulation and distribution in the South China North Basin.