Accumulation conditions of shale gas in transitional marine-continental coal measures of the Carboniferous-Permian in Henan Province

The Carboniferous-Permian strata in Henan Province are a set of coal-bearing series of transitional marine-continental facies,of which the mudstones have potentials for shale gas exploration.The mudstones are characterized by a variable thickness of the individual effective mudstone layer and a huge accumulative thickness.In this paper,we utilized the outcrop and borehole core data and investigated the accumulation conditions and the reservoir characteristics of the shale gas in the Carboniferous and Permian coal-bearing strata of Henan Province.The total organic carbon (TOC),organic matter types,thermal evolution degree,contents of brittle minerals and clay minerals,and pore morphological types of the mudstones were determined by the TOC analyzer,microscopic identification of kerogen,reflectance measurement of vitrinite,X-ray diffraction (XRD),low temperature nitrogen gas adsorption,respectively.The results showed that the abundance of organic matters of the mudstones was relatively high,with the TOC contents in the mudstones of the Taiyuan,Shanxi and Xiashihezi Formations being 0.95%～8.04%,0.78%～9.41% and 0.2%～2.13%,respectively.The types of organic matter were dominated by type Ⅲ,followed by type Ⅱ₂.The thermal evolution degree ranged from mature to over-mature stage which was favorable for the formation of dry gas.The maturity of the mudstones had a zoned distribution,with the highest maturity being distributed along the Jiaozuo-Zhoukou belt and the lower maturity being distributed towards the south and north from this belt.The XRD analysis showed that the clay minerals of the mudstones could account for 62.0% while the brittle minerals only accounted for 34.1%,indicating that the mudstones had a higher gas adsorption capacity but were difficult for hydraulic fracturing.The clay minerals were mainly composed of 57.1% of illite/smectite mixed layer and 23.5% of kaolinite,which were beneficial to the formation of pore space.The porosity of the mudstones ranged from 1.0% to 4.5%,and the permeability varied between 0.003 and 0.032 mD.The experiment of low temperature nitrogen gas adsorption revealed that the morphological types of the pores included the parallel plate-slit pores and the inclined plate-slit pores.The meso-pores provided the vast majority of specific surface area and total pore volume.The gas contents could reach 1.0～3.0 m³/t which is suitable for the condition of shale gas reservoir.It was concluded that the siliciclastic member in the middle Taiyuan Formation (H₁) and the basal part of the Shanxi Formation (H₂) have a higher hydrocarbon potential.The major favorable exploration areas are those having a good preservation condition or a less serious influence from the regional thermal events.