Multiphase tectonic movements, cap formations and evolution of the Majiang paleo-reservoir

The Majiang paleo-reservoir is a typical destroyed hydrocarbon reservoir, buried in carbonate strata of China＇s southern marine-facies. Field geological explorations, interpretations of seismic profiles and balanced cross-section restorations around this paleo-reservoir reveal that its formation and evolution have been restricted by multiphase tectonic movements of different intensities. A regional tectonic mechanism and model have been suggested for the formation and evolution of the Majiang paleo- reservoir. Geological field exploration has been carried out along three typical Silurian cross-sections and rock samples were tested in combination with water-rock interaction. Based on the result of cap tests, the planar distribution, the residual thickness, the erosion thickness and the preservation conditions, the Silurian mudstone cap is discussed around the Majiang paleo-reservoir. Combining the hydrodynamic conditions of its formation and evolution and its tectonic movements, we determined the fact that the thicker the cap is, the more resistant it is to hydrodynamic destruction. The multi-phase formation and destructive geological model of the paleo-reservoir is established through an overall analysis of multi- phase tectonic evolutions, cap developments, hydrodynamic conditions and solid mineral metallogenic ages measured by Rb-Sr, Pb and Sm-Nd isotope techniques.

Discovery of a new Proterozoic paleo-reservoir and its oil source in the Jianchang Sag,Yanliao Faulted-Depression Zone,North China

The discovery of many Proterozoic primary oil and gas reservoirs around the world testifies that the Meso-Neoproterozoic petroleum is an energy resources realm worthy of attention.This paper firstly reports the occurrence of hydrocarbon accumulations in the Neoproterozoic Luotuoling Formation in the Jianchang Sag,the Yanliao Faulted-Depression Zone.Petrography observation shows that the lower sandstone member of the Luotuoling Formation contains yellow fluorescent liquid hydrocarbons,while the upper sandstone member is rich in solid bitumen.Further analysis of the biomarkers and isotopic compositions reveals that the upper and lower reservoirs have different oil sources.The bitumen in the lower reservoir is rich in C_(19)-C_(20)tricyclic terpanes(TTs),13α(n-alkyl)-tricyclic terpanes and rearranged hopanes,but lacks gammacerane and steranes and depletes ^(13)C isotope(-33.4‰to-30.6‰).This is consistent with the properties of the Hongshuizhuang Formation source rocks.The upper reservoir bitumen has C_(23)TT as the major peak among C_(19)-C_(23)TTs,lacks 13α(n-alkyl)-tricyclic terpanes,has a low abundance of rearranged hopanes,obvious distribution of gammacerane and steranes,and depletion of the^(13)C isotope;essentially the same as the Gaoyuzhuang Formation source rocks.The discovery of oil seepage in the Luotuoling Formation of the Han-1 well is a promising indicator of the Precambrian oil and gas exploration potential of the Jianchang Sag.

Formation of Natural Bitumen and its Implication for Oil/gas Prospect in Dabashan Foreland

Natural bitumen is the evolutionary residue of hydrocarbon of sedimentary organic matter. Several kinds of bitumen with different occurrences, including bitumen in source rock, migration bitumen filled in fault, oil-bed bitumen and paleo-reservoir bitumen, are distributed widely in the Dabashan foreland. These kinds of bitumen represent the process of oil/gas formation, migration and accumulation in the region. Bitumen in source rock fiUed in fractures and stylolite and experienced deformation simultaneously together with source rock themselves. It indicated that oil/gas generation and expelling from source rock occurred under normal buried thermal conditions during prototype basin evolution stages prior to orogeny. Occurrences of bitumen in source rock indicated that paleo- reservoir formation conditions existed in the Dabashan foreland. Migration bitumen being widespread in the fault revealed that the fault was the main channel for oil/gas migration, which occurred synchronously with Jurassic foreland deformation. Oil-bed bitumen was the kind of pyrolysis bitumen that distributed in solution pores of reservoir rock in the Dabashan foreland depression, the northeastern Sichuan Basin. Geochemistry of oil-bed bitumen indicated that natural gas that accumulated in the Dabashan foreland depression formed from liquid hydrocarbon by pyrolysis process. However, paleo-reservior bitumen in the Dabashan forleland was the kind of degradation bitumen that formed from liquid hydrocarbon within the paleo-reservior by oxidation, alteration and other secondary changes due to paleo-reservior damage during tectonics in the Dabashan foreland. In combination with the tectonic evolution of the Dabashan foreland, it is proposed that the oil/gas generated, migrated and accumulated to form the paleo-reservoir during the Triassic Indosinian tectonic movement. Jurassic collision orogeny, the Yanshan tectonic movement, led to intracontinental orogeny of the Dabashan area accompanied by geofluid expelling and paleo-reservoir damage in the Dabashan foreland. The present work proposed that there is liquid hydrocarbon exploration potential in the Dabashan foreland, while there are prospects for the existence of natural gas in the Dabashan foreland depression.