Main Controlling Factors Of Gas Accumulation And Distribution In Igneous Rocks, Xujiaweizi Rift, North Songliao Basin

Ingtegrating with the study of regional geologic background, the thesis has full used the geology, seismic, well-logging and analysis data to systematically study the formation and development characteristics of igneous rock, genesis of gas, and the mechanism and model for igneous gas reservoir formation in Xujiaweizi Rift. The result shows that Xujiaweizi Rift is a half graben-like fault depression that consists of fault in the west and overlap in the east, and is controlled by the main boundering faults of Xuxi Fault and Xuzhong Fault which divide Xujiaweizi Rift into"two uplifts and three depressions"in structural pattern. The structural evolution of Xujiaweizi Rift has experienced three stages—incubation,development and fade—during which three structural compactions result in fault-bending fold, fault-propagation fold and inversion structure. These structures along with the extentional structure and accommodation structure caused by extensional movement provide the favorable trap belts for deep gas accumulation. Gas characteristics comparison inδ¹³C₁-δ¹³C₄, carbon isotope of light hydrocarbon, hydrogen isotope and the construction of light hydrocarbon in Xujiaweizi Rift proves that gas of igneous reservior is mainly from coal measure strata of Shahezi Formation(K₁sh), and part of it from the source rock of Huoshiling Formation(J₃–K₁h) and Yingcheng Formation(K₁yc). The igneous rocks in Xujiaweizi Rift mainly distributes in K₁yc1, K₁yc₃ and J₃–K₁h₁. Five kinds of facies and fifteen kinds of subfacies are classified according to the igneous rock facies classification scheme of lithology—texture—genesis. It can be surely believed that the favorable igneous rock facies belt are multi-facies overlapping zone near the volcanic crater and eruption-effusion facies overlapping zone far from the volcanic crater. In the light of genesis the igneous reservoir space is divided into primary pore, secondary pore and fracture.The most important types of pores are air hole, amygdale, intercrystalline pore and phanerocryst dissolved pore. The structural facture and explosive fracture are the most important fracture types. The favorable igneous reservoir mainly distributes along three fracture belts, which are Xuxi fracture belt, Songxi fracture belt and Yuxi fracture belt from west to east, like pinch and swell form in general direction of NNW or nearly north-south. The analysis of typical gas reservoirs in igneous rock of Shengping and Xingcheng shows that the type of gas reservoirs entirely is litho-structual, and that the igneous reservoir distributes in the following three sub-facies: the upper sub-facies of effusion facies, thermal debris flow sub-facies of explosion facies and cryptoexplosive breccia sub-facies of volcanic conduit facies, but serious are the reservoir heterogeneity. Both gas reservoirs belong to the normal temperature-pressure system, which don't have unified gas-water contact or pressure system, but the differential characteristics with gas upward and water downward. Formation and distribution of igneous gas reservoir in the Rift are mainly controlled by source rock distributed area, discordogenic fault, igneous rock facies, palaeohigh and igneous caprock of its own. Main periods of igneous gas reservoir formation are from the last stage of K₁q to the middle stage of K₁qn, the last stage of K₁n and the last stage of K₂m respectively. The models for igneous gas reservoir formation mainly include two types: Transported by the faults near source rock type and Transported by the faults far from source rock type.