Characteristics Of Exhalative Hydrothermal Rocks And Its Impaction On Oil And Gas, Permian, Santanghu Basin, Xinjiang

The Permian Lucaogou Formation (LF) is an important set of hydrocarbon source rock and reservoir in Santanghu basin, Xinjiang province. This layer is considered by predecessors to be composed of normal lacustrine clastic sediment and carbonate deposition. However, our research team recent study shows that the layer is mainly composed of very fine (0.5mm-1mm) laminated light-colored dolostones, analcime rocks, chert rocks, feldspar rocks, interlaminated with fine laminae of dark mudstones and tuff. And the laminated light-colored rocks are infrequent "white smokers" exhalative hydrothermal rocks (EHR) in an intracontinental lacustrine facies. Based on plenty of materials of academic papers at home and abroad, we choose laminated EHR in core at the central of the Santanghu basin as the research object. Combined with regional geology, sedimentary geology and petroleum geology, we analysis the features of mineralogy and petrology, the laws of vertical and horizontal distribution, forming mechanisms, and their impaction on oil and gas. Conclusions are summarized as follow.The Santanghu basin is a superimposed reconstruct intermontane basin, it has a pre-Permian basement and Permian-Quaternary sedimentary cover. The basin was a starved and deep lacustrine intracontinental rift basin containing mainly intraclastic deposits during the Middle Permian. The features that carbonate and sulfate fill in high angle, reticulate and arborization fractures are developed in underlying Upper Carboniferous volcanic rocks. Huge thick flood basalt, small amount of picrite and inhomogeneous distributed sedimentary rocks are developed in overlying Middle Permian Tiaohu Formation (TF), which shows that the lake basin sinked into atrophy and volcanic activity was frequent in Tiaohu time. The Middle Permian LF consists of laminated "white smoker" EHR interlaminated with dark-colored mudstone and tuff, which indicates a stable sedimentary environment in lake basin expanding period. The volcanic interlayer in LF shows a complete evolutionary series (basic rock to intermediate-acid rock):parlkaline basic cabonatite, parlkaline intermediate-acid analcime phonolite, intermediate-acid ignimbrite, which reflects a intracontinental rift extensional setting.Dolomite and ankerite occur as cement mainly exists in laminated dolostone. Analcime and alkali feldspar are considered to have been reformed from analcime. These clastics had experienced exhalative brecciation, transport, and deposition, then cemented by carbonate. Calcites float on the surface of other minral in form of mist trait, and parts of them are "porphyric texture" or form oolitic limestone. On the basis of mineral composition, the EHR mainly consists of dolomicrite and with less and dense micrite silicalite, analcimemudstone, K-feldsparmudstone and micrite in some layers. The EHR are commonly in characteristic of fine laminate (0.5-1mm) horizontal bedding, micro-cross bedding, micro-brecciated texture, whirl structure, syndepositional structure, and its grain size is mostly less than0.01mm, mainly anhedral. The strontium isotope of laminate dolostone is between0.7046-0.7062. Ankerite cements those clastics as cement, and anhedral dolomite occurs as both matrix and cement. Cathodoluminescence microscopy suggests that dolomite and ankerite formed earlier and calcite filled in dolostones’ residual pore and dissolved pore. Comprehensive research shows that the dolostones in study area are primary dolomite precipitated directly from mantle-originated hydrothermal fluids (MHF). Fischer-Tropsch process provides Mg2+and Fe2+which are necessary to form dolomite. The evidences of EHR are brought forth as fllows:①the distribution of dolomite in LF is closely related with the volcanic rocks in TF, which indicates the correlation between the channel of hydrothermal activites in Lucaogou time and volcanic activites in Tiaohu time;②the composition and distribution of EHR are very inhomogeneous, which reflects that different hydrothermal vents have different compositions of the hydrothermal fluids;③heavy mineral characteristics indicate various types and provenance;④the fluid inclusions in different depth of LF have the same homogenization temperature range;⑤Vrious types of flood vocanic rocks are interlaminated with exhalative sedimentary rocks.As the representative of exhalative rocks, formation mechanism of dolomite is discussed briefly, which is closely related to MHF. MHF migrated upward along the fault, which was caused by magmatic activities, and ejected in the bottom of the lake. Carbonate hydrothermal provides the CO32-and Ca2+,reacts with the wall rock at the same time and brings the cationic from the wall rock. The fluid from the upper mantle caused serpentinization of ultramafic rocks that intruded into the lower crust to release Mg2+and Fe2+, which injected the ions into the lake water as the Mg and Fe sources formed exhalative dolostone. Near the vent, coarse brecciated clastics is cemented by ankerite which is precipitated directly from MHF. Far away from the vent, deposits form relatively pure laminated dolomicrites with small amount of mud-sized.Hydrothermal activity and associated thermophilic bacteria provide the foundation of the continuity of food chain maintaining in this special environment that accumulate organic matter, which is the material basis for oil and gas forming. Mantle-originated material is likely to bring mantle-originated hydrocarbons. Moreover, hydrothermal activity can also increased the geotemperature of sedimentary basin that may accelerate the maturity of oil and gas.