The Shale Gas Exploration Prospect Of The Lower Paleaeozoic In Western Hubei And Hunan-Eastern Chongqing

The upper Yangtze area is the strategic area of shale gas exploration in China, itstarget strata is the lower Palaeozoic sequence including the lower Cambrian Niutitangformation, upper Ordovician Wufeng formation and lower Silurian Longmaxiformation. The western Hubei and Hunan-eastern Chongqing area locates in thesoutheast margin of upper Yangtze area. In this paper, based on sedimentology,petrology and lithology etc, we have studied the geological condition, accumulationcondition of shale gas in western Hubei and Hunan-eastern Chongqing area, tounravel its exploration prospects.There are different provenance for the lower Cambrian Niutitang formation, andthe upper Ordovician Wufeng-lower Silurian Longmaxi formation, they are fromHannan and northern Sichuan paleo-high, and from Hannan and central Sichuanpaleo-high, respectively. The rock types of Niutitang formation are dominant withblack shale, silty shale and argillaceous siltstone, deposited in the facies deep-watershelf, shallow continental shelf, marginal slope. However, the rock types of Wufeng-Longmaxi formation are black shale, siliceous and silty shale, argillaceous siltstone,deposited in the facies of deep-water shelf, neritic deep-water basin and marginalslope. In particular, there is widespread graptolite in Wufeng formation.It is characterized with high-quality source rocks in Niutitang formation, andWufeng-Longmaxi formation, its best locates in Shizhu and Xiushan area. The sourcerock is60¹20m thick, dominant with I-II types of kerogen, and high organic contents（TOC ranging from2.0%to4.0%）. However, the Ro of source rock ranges from2.0%to5.0%, probably related to the overlying strata with significant thickness, andtectono-thermal events in region.The type of reservoir pore in lower Palaeozoic shale includes lamellar micropore,intergranular micropore and intercrystalline pore, derived from the unstable mineraldissolution, mineral alteration during the diagenesis, organic matter generation. Due tomulti-tectonic events, in particular the rapid uplift and exhumation during Cenozoictimes, the lower Palaeozoic shale experienced strong desorption of adsorbed gas, anddiffusion of free gas. Therefore, the content of shale gas in our studied area is muchsmaller than the Barnett shale in American, with a content of1.0³.0m³/t. Thus, the possible and probable resources of Niutitang formation are3.32×10¹²m³and0.39×10¹²m³, and the total and probable resources of lower Silurianare3.95×10¹²m³and0.55×10¹²m³. We suggest that such areas with good condition ofsource, weak tectonic movement and shallow buried depth, could have the bestexploration prospects of shale gas in western Hubei and Hunan-eastern Chongqing.