Reservoir Characterization Of The Marine-continental Transitional Tight Sandstones In The Longtan Fomation,West Guizhou,China

Unconventional resources,such as tight sand gas,shale gas,and coalbed methane have become important parts of natural gas supply to meet global energy needs.The Upper Permian Longtan Formation was formed in marine-continental transitional environments,and thus the resource potentials for exploration and development of tight sandstone gas in the Longtan Formation are very attractive.This paper takes the Longtan sandstones in west Guizhou as an example to investigate the marine-continental transitional tight sandstones by using various techniques(e.g.,thin sections,scanning electron microscopy,X-ray diffraction,mercury injection capillary pressure,nuclear magnetic resonance,fluid inclusion,and noble gas isotope measurement)to illustrate the reservoir characteristics and their mechanisms of the Longtan sandstones,to assess the filling time and main source contribution of tight gas,and to discuss their impacts on the tight-sand gas potential.The Longtan sandstones deposited in delta and tidal flat–lagoon environments,according to our study,are classified as litharenites with relatively low compositional and textural maturities.The Longtan sandstones are characterized by high rock fragment and clay contents,low porosities and permeabilities,abundant micropores,and high clay-bound water saturations.Compared with other typical tight sandstones in China(e.g.,the Yanchang,Xujiahe,and Bashijiqike sandstones),the Longtan sandstones have greater lithic contents and tighter pore throat structures.Furthermore,the fractal analyses of mercury injection capillary pressure and nuclear magnetic resonance data reveal that the Longtan sandstones have large fractal dimensions which signify complex and heterogeneous pore throat structures.The qualitative and quantitative evaluation of the diagenesis of the Longtan sandstones indicates that the comprehensive diagenetic index(CDI)can characterize the integrated impact of compaction,cementation,dissolution,and micro-fracturing on the pore throat structure of the reservoir.The strong destructive(compaction and cementation)and weak constructive(dissolution and micro-fracturing)diagenesis effects during the burial history result in low CDI values and tight pore throat structures of the Longtan sandstones.Furthermore,the comprehensive reservoir evaluation index(CREI)was established by using the gray correlation method to incorporate reservoir properties,such as macroscopic reservoir properties,microscopic pore throat structure characteristics,diagenesis impacts,and heterogeneities to determine three different reservoir types of the Longtan sandstones.The percentage of high-quality type I reservoir in the Longtan Formation is low,and type II and III reservoirs are widely distributed in the study area.Overall,the Longtan sandstones have tight pore throat structures,poor reservoir qualities,and strong heterogeneities.The fluid inclusions within the Longtan tight sandstones in this study are categorized as primary gas-liquid,secondary brine,and secondary gas-liquid inclusions with the components of methane,saturated hydrocarbons,and carbon dioxide,indicating that gases have been accumulated within the tight sandstones.Their homogenization temperatures combined with the burial and thermal histories show that the Longtan tight-sand gas is a continuous charging accumulation which occurred during the Late Triassic.In addition,studies on the contents of noble gas isotopes(He and Ar)infer that the tight-sand gas mainly comes from shales of the Longtan Formation,but the coal seams contribute very little.Therefore,high TOC contents and appropriate maturities of shales can generate sufficient hydrocarbon to charge neighboring sandstones to form tight-gas accumulation in the Longtan Formation.The southwest of the study area is a potential area for tight sandstone gas development,since the tectonic destructions are relatively weak,the structural depths are relatively shallow,the sand thicknesses and net gross ratios are large,and the reservoir qualities are relatively good.Vertically,sandstones are interbedded with shales and coal seams.A model for commingled production of tight-sand gas,shale gas,and coalbed methane could be viable,which provides an opportunity for unconventional gas exploration in west Guizhou.