On The Mechanisms And Models Of Petroleum Secondary Migration In Dongying Depression

1. IntroductionWe share two rather sophisticated and mature theories accounting for the secondary migration and accumulation of petroleum: One is the theory of fluid potential put forward by Hubbert M.K.(1940. 1953) and modified by England W.A.(1987), explaining the lateral secondary migration along a unique aquifer, suggestive of a lateral secondary migration of oil and gas driven by buoyancy and hydrodynamic force; The other is the theory of vertical secondary migration along faults and fractures from deep, overpressured and compartmented fluid systems upward to shallow hydrostatic systems during repeated seal rupture in geologic time, which was proposed by Hunt J.M.in 1990. In this paper both the two theories are used to explain the secondary migration and accumulation of petroleum in Dongying depression. By detailed analysis of structure, hydrodynamics and hydrocarbon distribution in the depression, new mechanisms and models for oil and gas secondary migration and accumulation in this basin is developed, which will suggest a more accurate conceptual model for future study such as quantitative simulation in this kind of basins.2. Geological Outline of Dongying DepressionDongying depression is a Meso-Cenozoic rift-subsident basin, with platform cap formation of the Paleozoic era as its basement. It experienced 3 stages in its evolution: (1) prototype stage —early rift-subsidence during the late Jurassic and Creataceous, which resulted in small grabens filled with terristrial clastic sediments and neutral-basal volcanic rocks; (2) stage of strong rifting and down-warping in the Paleogene, during which the depression subsided fastly along the discordogenic border fault in the north edge, a half-graben basin formed and filled with thick lacustrine and fluvial sediments characterized by its high sedimentation rate, intensified rifting and fracturing and large amount of growth faults in the Eogene; (3) stage of down-warping, which is characterized by minor fracturing and overall subsidence of the whole Bohaibay basin and widespread alluvial and fluvial deposition. The main target formation for petroleum exploration in this depression is the Eogene and Neogene. the thickness of which ranges from several hundreds of meters to over 7000 meters, and lithology of which is terrigeneous sandstones, siltstones and shales with a little carbonates and evaporites.Structures inside the Dongying depression, especially in Eogene are controlled by fracturing. The depression experienced a long period of rifting and developed large amount of faults of different orders: the first order faults are boundary of depression, the second order ones...