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Hydrocarbon Accumulations In and Around Basalts in the Cenozoic of Northeast Bohai Bay Basin, China


As the most wide-spread extrusive volcanic rocks, reservoir properties of basalts are considerable due to vesicles and contraction cracks and fractures, which is significant to reserving of hydrocarbons, ground water and heat resources, and also to permanent carbon dioxide storage. Commercial oil and gas reserves had been found in the deep part of major basins in China including Bohai Bay, Songliao and Junggar. In Liaohe Depression of Bohai Bay Basin, Cenozoic basalts with a total thickness over 1000 meters revealed by hundreds of boreholes were primarily formed in subaerial surroundings. Based on observations and descriptions of drilled cores with a total length of 673 meters from 60 boreholes, resin-impregnated thin section and scanning electron microscope analysis, mercury injection and porosity & permeability tests are used to characterization of pore networks and evaluation of reservoir properties. Multiple pore network of basalts is composed of diverse pores and fractures with different scales ranging from nanometers to tens of millimeters. Porosities and permeabilities of basaltic rocks are highly heterogeneous due to the zonation of vesicles, fractures and brecciations. Overall, porosities range from 0.9% to 30.2%, and permeabilities are between 0.01×10-3μm2 to 36.6×10-3μm2. Four subcategories of basalts were divided by their textural diversities, respectively vesicular, brecciated, fractured and massive basalts. Variances of porosities and permeabilities among these four subcategories shows the highest porosity in vesicular basalt (avg. 13.4%), medium porosity in brecciated basalt (avg. 11.7%) and the lowest porosity in massive basalt (avg. 8.3%), and highest permeabilities are revealed by fractured basalts. Moreover, dissolutions related boundaries of lava flow units and fracturing by multi-stage of faulting activities are significant to the enhancement of secondary porosities and connectivity of pore networks. Hydrocarbon shows indicate that oil and gas are preferentially accumulated underneath the boudaries of basaltic lava flow units. The highest oil production by single borehole is amount to 33.4 m3 per day. Three genetic types of lava flow units are dominant in the hydrocarbon bearing basalts, including the compound-braided flow, tabular flow and hyaloclastite. Reservoir properties and hydrocarbon shows are favourable in compound lava flows and hyaloclastites, which should be taken as the primary hydrocarbon exploration targets.