Formation Mechanism Of Volcanic Reservoir In The Post-Collisional Setting

There are widespread carboniferous volcanic rocks in Junggar basin. Recently, an important industrial gas has been developed from the Bashan formation volcanic rocks in Dixi area nearby Kelameili Mountain, which reveals a bright future for exploration in these volcanic rocks.Based on total420.5m drilling cores inspection in10wells and more than1000thin section analyses in Dixi area, we found that types of carboniferous volcanic rocks in Dixi area include mainly subvolcano rocks, volcanic lava, volcaniclastic rocks and volcanic sedimentary rocks, which subdivided into syenite porphyry, basalt, trachyte, volcanic breccia, tuff, sed-volcanic breccia and sedimentary tuff. The characteristics of major elements, trace elements and REE in the volcanic lava and subvolcano rocks show that basalt, trachyte and syenite porphyry have the same or similar REE patterns and combination feature of trace elements, which suggest that they are homology. Magma may originate from the depleted mantle which mixed with lower crustal material. Basaltic magma may have experienced the fractional crystallization of pyroxene, plagioclase, ilmenite, apatite and other minerals to form trachyte. Consociated with the fact that syenite porphyry invade repeatedly into trachyte, trachytic volcanic breccia and sedimentary tuff, it is shown that trachyte belongs to subvolcanic body.This paper reports a343.2±4.0Ma age of syenite porphyry by LA-ICP-MS zircon U-Pb geochronology. It can be concluded that the time of Bashan formation is Visean in the early carboniferous, which is after the time about the closure of Kalamaili Ocean. Combined with the characteristic of trace element, the volcanic rocks formed under the post-collisional setting between collision and intra-plate period. Three stages of volcanic activity broke out under the post-collision extension setting, forming volcanic rocks series of quite overflow and strong outbreak, also with ultra-shallow intrusive. From the northwest to the southeast, types of volcanic rock change from the mafic gradually to the acid. Their distribution in space show that the post-collisional magmatism has the successive, periodical and orientational properties.According to the influencing factors such as environment and intensity of eruption, and in combination with texture and structure of rocks, following the evolution features of magma at different stages, the formation and evolution of primary reservoir spaces is divided into four stages:"magma chamber" stage, outpouring surface stage, diagenetic stage of condensation and deuteric hydrothermal stages. It’s believed that the cleavage crack and twinning striation of crystallized phenocryst minerals in "magma chamber" are the foundation of primary and secondary reservoir spaces. The outpouring surface stage and diagenetic stage of condensation are the main periods to form primary apertures, and the speed of crystallization plays a key role in the formation of different types of primary reservoir spaces. The hydrothermal stage reduces the primary apertures by the filling of low-temperature minerals, but the products are the best reshaped objects during the secondary processes.Combined with the history of burial, hydrocarbon generation and tectonic evolution, the evolution rules of reservoir spaces in subvolcano rock, volcanic lava and volcaniclastic rock is found as follows: Several times of extension and extrusion under post-collisional setting in middle hercynian formed the multi-stage characteristics of volcanism. And this is the premise of volcanic rock undergoing multiple exposures with weathering and leaching. The first mature time of source rocks in early Yanshanian and the saturated hydrocarbons cracking into gas in middle-late Yanshanian was the main stage of forming secondary reservoir spaces with a large number of dissolution. Structural fracture formed in Indo-china and Yanshan tectonic movement was the key factor to constitute the excellent combination of volcanic reservoir space.Statistics characteristics of physical property and pore configuration show that the volcanic rock reservoir is middle-higher in porosity and low-lower in permeability in Dixi area. The pore configuration in pyroclastic rocks and subvolcano rock is the best, followed by volcanic lava, and volcanic sedimentary rock is the worst. According to the standard of reservoir classification evaluation, the most developed one is type Ⅲ (approximately57.75%), type I is in a very small amount (only4.53%) and type Ⅱ is between them (37.72%). The characteristic of reservior variation in transverse reveals that there are mainly type III distributing in middle-upper part of C1b3-in Dixi17well area, and its distribution is controlled by the lithology and lithofacies. The reservior in Dixi14well area is mainly distributing in C1b22and middle-upper part of C1b11,showing type Ⅱ reservior and continuous layered distribution in transverse. In Dixi18well area, the reservior controlled by the intrusive body and fault is mainly type III distributing in C1b21.In Dixi10well area, the reservior is mainly type II and III distributing in C1b31, middle-upper part of C1b32and the upper part of C1b21. Various types of reservoir on the plane show the characteristic of isolated and not-contiguous distribution.The volcanic rock reservior in extension and extrusion under post-collisional setting is unique. The formation and evolution of Carboniferous volcanic rock reservior in Dixi area is controlled by the background of tectonic evolution and the types of tectonism. Investigation in volcanic reservoir formation mechanism from the volcanic rock tectonic setting can reveal the distribution and discipline of reservoir, can perfect the theory of volcanic reservoirs, and can also provide a reference for the same type of oil-gas reservoir exploration and development in some other areas.