Distribution and origin of anomalously high permeability zones in Weizhou formation, Weizhou 12-X oilfield, Weixinan Sag, China

Abstract

In order to study the dominant seepage channel of the third member of Weizhou formation (Ew3) in Weizhou 12-X oilfield, Weixinan Sag, Beibu Gulf Basin, and tap the potential of remaining oil. The distribution and causes of the anomalously high permeability zones in Weizhou Formation were studied by using conventional core physical property analysis, scanning electron microscope, laser particle size analysis, X-ray diffraction and thin section microscopic identification. As the results show, vertically,there are three anomalously high permeability zones in the A11, A12 and A21 micro-stage of the middle diagenetic stage, with the depth range of 2300 m ~ 2400 m, 2400 m ~ 2600 m, 2600 m ~ 2900 m respectively. Grain size, sorting, dissolution and early emplacement of hydrocarbons are the main causes of anomalously high permeability zones. Although both grain size and sorting affect porosity and permeability, the effect of grain size on permeability is stronger than sorting, and sorting has a stronger effect on porosity than grain size. Magmatic hydrothermal and organic acid promote dissolution and concomitant porosity and permeability increase by dissolving unstable minerals. The early emplacement of hydrocarbons retard the cementation and accompanying porosity and permeability reduction by reducing the water-rock ratio. Finally, sandstone reservoirs in the Ew3 are characterized by anomalously high permeability zones.