| Permian organic-rich shales occur throughout the Lower Yangtze region in South China and are potentially a future target for shale gas exploration. However, the research level is very low, especially models examining reservoir pore structure and gas-in-place have been lacking for the Permian shale in Lower Yangtze region. The potential of shale gas resource is not implemented, which affects the process of exploration and development. In this study, a suite of Permian shale was collected from two exploratory wells(C1# and H1#) at Wuhu blocks and their geochemistry, reservoir properties, adsorption capacity and the evaluation of GIP(gas-in-place) were investigated. The evolution of pore structure and its influencing factors were studied throught thermal simulation samples. Combined with the geological parameters of the Permian shale in Lower Yangtze region, geological models of GIP were constructed and the potential of shale exploration and development was discussed. The following knowledge and results are obtained:(1) The seclected Permian shale samples have a TOC(total organic carbon) content of 0.6%-17.4% with an average of 7.8% and the organic matter is dominated by type III kerogen with a Ro(vitrinite reflectance) value ranging from 2.34% to 2.66%. Total porosity of the shale samples ranges between 0.98% and 5.52%. With the increase of TOC content, total porosity increases first and then decreased, and the turning point is approximately 12%. The decrease of porosity for high TOC content samples(> 12%) is related to the compaction of larger organic pores.(2) The differences of TOC content have an important influence on pore structure of shale. The micropore volume and surface area as well as mesopores surface area for the all samples, and the mesopores volume for samples with TOC < 12% display positive correlations with TOC content, whereas the mesopores and macropores volumes for the higher TOC samples(> 12%) are negatively correlated with TOC content. This may imply that compaction is mainly expressed in the macropore and larger mesepore in organic matter.(3) The residual SOM(soluble organic matter) in the low mature shale has an important influence on the measurement and characterization of pore structure. The residual SOM in the shale samples occur mainly in the micropores and small mesepores, and their occupied pore size seems being constrained by the maturity. For the shale samples with lower maturity, SOM is mainly hosted in pores with diameter less than 5 nm. For the shale samples with moderate maturity, micropores and mesopores ranging between 2 and 20 nm are the main storage space for SOM.(4) The adsorption parameters(e.g., the maximum excess and absolute amount, the adsorbed phase density) of Permian shale are mainly constrained by TOC and adsorption temperature. They are positively related to TOC content, but adversely to adsorption temperature. Based on SDR equation, geological models of GIP were constructed. With increasing depth, the excess and absolute adsorbed capacity shows at first a rapid increase and then an obvious decrease with a maximum value occurring at depths of 400–700 m and 900–1,400 m, respectively. However, the free gas content shows an increasing trend.(5) The models of GIP for different TOC content shale were established, taking the research region as an example. The TOC content is the key factor to control gas storage capacity and occurrence state. The gas storage potential of samples with low TOC(< 4%) is poor, because both the adsorbed and free gas amount is low. Samples with very high TOC content(> 12%) have relative low storage capacity for free gas. The percentage of free gas is low, which affects the development of shale gas. Deeper overpressure reservoir is the exploration target of the Permian shale in Lower Yangtze region. |
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