RESERVOIR HETEROGENEITY OF THE LIUSHAGANG FORMATION AND ITS SIGNIFICANCE IN THE W AREA OF SOUTHWEST WEIZHOU DEPRESSION OF BEIBU GULF BASIN

Detailed microfacies study and statistical analysis of fine well logging data are carried out for reservoir heterogeneity of the Liushagang Formation in the W area of the Southwest Weizhou Depression of the Beibu Gulf Basin. So far there have found four oilfields in the W area namely WA, WB, WC and WD respectively. Reservoirs mainly occur in the Member 1 and 3 of the Liushagang Formation. Sedimentary microfacies of the interlayers of the reservoir suggests that the Liushagang Formation is composed of normal delta front and fan delta front deposits including distributary channel depositss, sand bar deposits, barrier depositss and braided channel sandstones and mudstones. Sedimentology of the fan deltas is studied upon the basis of fine stratigraphic division and correlation. It shows that the interlayers in the Member 1 are dominated by mudstone, silty mudstone with argillaceous silty sandstone and some thin tight layers of sandstone. The interlayer is mainly formed in argillaceous depositional environments between the fluvial deltaic channels or between underwater distributary channels, and/or between sandy bars. Through the comparative study of reservoir heterogeneity based on microfacies, the spatial change in physical properties of reservoirs is revealed. From the statistical results of the fine-interpreted data, it is observed that the reservoirs in the Member 1 of Liushagang Formation in the WA Oilfield has more and thicker interlayers and higher heterogeneity in general; the WB Oilfield has uniformly distributed interlayers, heterogeneity is low in the southeast and high in the northwest. The Member 3 of Liushagang Formation in the oilfields WC and WD have abundant interlayers, but heterogeneity is low in general. Based on the study results of reservoir heterogeneity, prediction is made for the remaining oil of the main reservoir layers in the WA Oilfield. Then two adjustment wells were deployed to tap the remaining oil. The cumulative production is increased for 4.2×10⁴ t, about 2.3% of enhanced oil recovery. This successful application shows that interlayer and plane heterogeneity study has provided solid scientific foundation for tapping the remaining oil out. It is of great significance for efficient oilfield development.