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Residual oil Characterization and Influencing Factors in NgⅠ-2-2 and NgⅢ-1-1 Reservoir Groups of Guantao Formation in Second District, Gangxi Oilfield, China

Abstract

The second district of Gangxi Oilfield in China belongs to a part of north Gangxi drape anticline surrounded by a fault trending north-east in the south area. NgⅠ-2-2 and NgⅢ-1-1 are the major reservoir groups of this region. The average percent recovery is about 13% with high water cut of 91%. The objective of this study is to characterize the distribution of the residual oil of NgⅠ-2-2 and NgⅢ-1-1 and explain the influencing factors of the residual oil. This study is carried out by refined stratigraphic division, characterized profile and sedimentary facies and sand bodies of major reservoirs. As for 12m thick NgⅠ-2-2 reservoir group, the sedimentary environment is braided river with source from north-east direction. Braided bars are widely developed and overlapped several time. Narrow channel facies separate the braided bars apart. Edge water reservoirs are more developed than bottom water reservoirs. The percent recovery is 34% with the residual oil mainly distributed around the higher structural position near north-east fault. Small amount of residual oil is found near production wells. For 22m thick NgⅢ-1-1 reservoir group, edge and bottom water reservoirs are widely developed with huge amount of reserved petroleum. Oil has high viscosity of 2.41×103mPa·s, which is caused by intense oxidation and the percent recovery is only 6.36%. The distribution of the residual oil is wide, not limited to the area near fault and large reserves of oil can be exploited. Geological and production factors influence the distribution of the residual oil. Geological factor includes tectonics, sedimentology and fluid properties. Area near fault and relatively higher structural position are suitable for residual oil. For braided river sediment environment, less residual oil distribute along the channel direction because the sediment has higher porosity and better permeability. Interlayer siltstone and mudstone has certain influence on delaying or stopping bottom water conning. High-viscosity oil is easier to cause water conning. As for production factor, more residual oil centralized near area with less injection wells and less production wells.