Research On Genetic Feature Of Organic Fluid In Foreland Structure Of Daba Mountains

Organic fluid is the original records of structural evolutions and migration activities of oil and gas. Formation and movement features about organic fluid in foreland of Daba Mountains have been discussed according to the macroscopic investigations outside and laboratory analysis. The methods of organic geochemistry and carbon-hydrogan isotope in methane from fluid inclusions are used during the study of formation of organic fluid.The results obtained are contributions to a better understanding of formation,migration and accumulation,thus,providing new clues to oil exploration in foreland of Daba Mountains.The foreland of Daba Mountains resulted from the intracontinental orogeny of the Qingling orogenetic movements. A series of nearly NEE-SWW direction thrust structure belts and faults were formed in the control of nearly north-south compression stress field.Meanwhile, the sedimentary system had been changed into continental basin sediment from marine carbornatite sediment.During J3-K1 (about 140Ma) Daba Mountains were turned into nearly east-west compression from nearly north-south compression,which had greatly transformed the former structural features.As a result,a series of tightly arranged long and narrow folded belts were formed, with the direction of NNW-nearly EW and WS-nearly EW.Source rocks are widely developed in foreland of Daba Mountains.By the study of organic geochemistry analysis,such as organic carbon, rock pyrolysis analysis,we conclude that research area are provided with abundant material basis for hydrocarbon generation and advantageous conditions. By the study of the thermal evolution and hydrocarbon generation,we consider that Paleozoic erathem source rocks in foreland of Daba Mountains have experienced times of hydrocarbon generation and expulsion during late Sinian to late Triassic,and fit the origination of paleo-reservoir. Natural gas pools were destroyed because of the orogenetic movements in Indochina period in the early stageof Lower Jurassic(about 180Ma),and finally were turned into a gas reservoir owing to the transformation of Yanshan movements in the Lower Cretaceous(about 136Ma-128Ma).Large numbers of hydrocarbon inclusions are found in the calcite veins and quartz veins in the faulted zones in foreland of Daba Mountains.Through the research on the carbon-hydrogen isotope in methane, we conclude that the discussed gas has a mix character of both oil-type gas and coal-related gas,meanwhile,faulted zones are the primary migration pathways.