| Natural gas hydrate (NGH) of permafrost area is the favor of clean energy researchers due to its considerable number and more easily mining features than Marine gas hydrates. But compared with abroad, researches on land hydrate reservoiring geological background, sedimentary environment of hydrocarbon source rocks, hydrate formation conditions, etc. in our country started late. Qinghai-Tibet plateau is one of the areas in the world that develop the biggest scope permafrost with most thick layer. The northern part of Qiangtang petroleum sedimentary basin has wide distribution of hydrocarbon source rocks of late Paleozoic to early Mesozoic, which can provide the necessary hydrate alkane gas for the formation of the NGH. Meanwhile, the preliminary prediction of the NGH shows that the Qinghai-Tibetan plateau have metallogenic conditions and prospecting potential of the NGH.This paper takes the forming geological background of the hydrocarbon source rocks and the forming condition of the NGH in Tuotuohe area, Qinghai-Tibet plateau region as the main research objects, in view of geologic evolution of the coal-bearing rock series before and after the formation sedimentary period in the study area, the sedimentary faces of the upper Permian coal-bearing rock series and the organic geochemistry systematically of hydrocarbon source rocks, together with selection evaluation of the hydrocarbon source rocks and comprehensive analysis of hydrocarbon gas composition and genesis, get the following understandings:1. The Tuotuohe area in Qinghai province develops a set of late Permian coal-bearing rock series stratum, sedimentary thickness> 1800 m. The land and sea sedimentary is the overall performance of the strata. Barrier coast and delta sedimentary system are developed in Wuli area, respectively by radiolarian siliceous rock and coal seam as their marks. The sedimentary faces of organic matter-rich lithologic section of lower strata is the delta plain faces, organic matter-poor lithologic section of upper strata is in the sedimentary environment of shallow water tidal flat faces. Microfacies control the development and distribution of coal. The development and distribution of microfacies control coal. Coal accumulating environment is peat bog, the main coal forming period is at the bottom of the Wujiaping-changxing order, Lopingian Series, with higher coal rank. Sedimentary environment and the formation of source rocks have a coupling relationships with NGH mineralization.2. Determination of the tectonic affinities of the late Devonian high-alumina basalt, the early Permian boninite, the late Permian alkaline - alkali tholeiite and the late Triassic calc-alkaline basalt show that the study area has a clear tectonic evolutionary context from the ocean (D2), subduction initiation (P1), epicontinental arc (P3) to mature (T3), the Permian(P3) carbon-rich mud-swamp hydrocarbon source rocks developed on the relatively mature and stable intracontinental environment, conducive to the prosperity of terrestrial creatures and preservation.Through the analysis of volcanism influence on the development of hydrocarbon source rocks, it is determined that a large number of reducing gases such as H2S, SO2 erupted from late Permian volcanic magma underwater are favorable for organic matter preservation, and the strata is parent material layer that rich in terrestrial and aquatic organisms developed with terrigenous clastic sedimentary. Late Triassic volcanic rocks have the dual effects of lower the effective carbon content of the upper Permian hydrocarbon source rocks and becoming the hydrate reservoiring blocks.3. The hydrocarbon source rocks in upper Permian have suffered lower regional metamorphism and dynamic metamorphism, metamorphic temperature of the former promote the maturity of organic matter in hydrocarbon source rocks, at the same time, strong adsorption of the new metamorphic minerals in rocks, such as the sericite, chlorite etc. strengthen the classifies ability of free methane gas; the tectonic stress of the free methane gas makes passive hydrocarbon expulsion of the hydrocarbon source rocks, and forms a large number of fractures or joint in the hydrocarbon source rocks, improves the pore permeability condition and reservoir space, and is conducive to the formation of hydrate in trap good tectonic position.4. On the analysis of the characteristics, the geologic process and melting permafrost area, it is found that permafrost geothermal gradient of the NGH formation in Tuotuohe area is close to that of the known land hydrate at home and abroad, meet the hydrate formation conditions of ground temperature. The existing perennial frozen earth layer in the study area mainly formed in the Everest ice age; the fractures in structure-melting zone is a combination of hydrocarbon gas migration channel.5. Based on the geological characteristics of hydrocarbon source rocks, it figures out that the coal quality of coal-bearing source rocks in the upper Permian Nayixiong group in the Tuotuohe area, is anthracite coal, with the high ash content and low metamorphic grade, and had experienced coal pyrolysis gas in the main stage. It is also found that the hydrocarbon source rocks are widespread in Wuli area, with great sedimentary thickness, moderate later tectonic activities, which are the favorable geological conditions for the hydrate formation.6. By rock pyrolysis analysis of coal-bearing rock series of the Permian Nayixiong group, it shows of the rock TOC (%)> 0.4 generally, Tmax (℃) between 408～555. Content of chloroform bitumen "A" is low, saturated hydrocarbon ratio reaches 33.45～88.17%. Organic matter types are mainly sapropel humic type (Ⅱ 2) and the minority humic type (Ⅲ). The largest vitrinite reflectance average is 3.41%, the hydrocarbon source rocks are in high maturity-overmature. Based on the analysis of biological marker compound and sedimentary environment, we propose that the hydrocarbon source rock organic matter from the upper Permian Nayixiong group derives from the mixture of terrestrial higher plants and aquatic organisms. The water which sedimentary stronger than the reduction and with high salinity, favors the preservation of organic matter. The most favorable hydrocarbon source rocks in upper Permian are coal-bearing rock series, the evaluation of the hydrocarbon source rocks is medium to good level.7. Through the differences of the geophysical properties of rocks, the logging curve on permafrost contains mudstone, coal and rock seam to explain and high ash content of coal strata, the paper effectively identifies multiple coal seams more than 51.55 meters in deep, coal and carbonaceous mudstone are parent material layer of hydrocarbon generation of coal-bearing rock series hydrocarbon source rocks.8. The formation conditions of the NGH in Tuotuhe area are:suitable temperature geothermal gradient, natural surface, thicker planar permafrost, containing coal seam, sufficient gas source, low intensity of fracture structure and good reservoir and cap rock combination. Hydrocarbon source rocks in coal seam control hydrate gas source in Nayixiong group. Methane is more than 99% in total gas composition, and is the mixture of pyrolysis gas and biogas. Dry coefficient reflects the hydrocarbon gas is dry gas, very conducive to the formation of hydrate.9. this paper systematically analyzes reservoir - cap reservoiring geological conditions of the NGH in the Tuotuohe area, established four modes of gas hydrate accumulation (fold trap, fault and lithologic trap, unconformity surface trap and permafrost trap); summarizes the four factors influencing the hydrate accumulation (plateau uplift, active fault, melting zone and the climate warming) and escaping ways of alkane gas, the changing of Quaternary climate and the thickness increase or decrease of the permafrost in the region show the coupling relationship with the saving of the NGH.10. With comparison of gas hydrate reservoiring pattern in Muli basin, Qilian Mountain, it puts forward that the hydrate reservoiring evolution in Tuotuohe area has experienced the stages of sedimentary organic matter preserved, hydrocarbon source rock diagenesis and metamorphism, hydrocarbon expulsion stage and hydrate reservoiring.11. According to the reservoiring geological conditions and organic geochemical parameters of the NGH in Kaixinling and Wuli area in Tuotuohe region, three favorable metallogenic zones and three favorable exploration target area of gas hydrate are delineated, which provide a scientific basis for the objective evaluation in the gas hydrate resource potential and exploration of the NGH in this region. |
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