Predicting potential exploration targets based on key factors controlling hydrocarbon accumulation-a case study from the Lower Ordovician Yingshan Formation in the Tazhong area, Tarim Basin, NW China

摘要

The Tazhong area, at the centre of the Tarim Basin, northwestern China, contains abundant hydrocarbon resources and features complex geological setting, principally in the Lower Ordovician carbonate reservoirs. Despite years of exploration and development, the hydrocarbon accumulation mechanisms and the key factors controlling hydrocarbon accumulation are still unclear. This lack of knowledge is the critical factor restricting hydrocarbon exploration in the Lower Ordovician Yingshan Formation. An analysis of the geological setting and reservoir distribution characteristics, the accumulation mechanisms and the key factors are investigated to predict the target exploration area. Our results show that the hydrocarbons, which experienced four stages of accumulation, originated from mixed sources including Cambrian-Lower Ordovician and Middle-Upper Ordovician source rocks within the Tazhong area and beyond. Hydrocarbons, by point charging, migrated into weathering crust karst reservoirs along 10 intersections of northeast and northwest fault sets, and then laterally migrated, principally in the NW-SE direction. The Yingshan Formation carbonate reservoirs are typically lithologically defined and primarily controlled by differential capillary forces. The accumulation and distribution of hydrocarbons in the Yingshan Formation were controlled by excellent reservoirs and hydrocarbon charge energy. The physical properties of excellent reservoirs control the petroliferous features and the hydrocarbon accumulation threshold: these reservoirs have a threshold with the minimum porosity and permeability, approximately 1.8% and 0.1 mD, and have a threshold with the maximum capillary forces, approximately 2.5 MPa. Hydrocarbon charge energy controls the hydrocarbon enrichment range: hydrocarbons enriched in areas near injection points and the hydrocarbon charge distance threshold of 20 km. Excellent reservoirs and hydrocarbon charge energy coupled index (RSI) controls hydrocarbon accumulation and distribution. Accumulation does not occur when the value of RSI is <0.5 but is favoured when values are higher. The coupled index is used to predict that lithological reservoirs in the Yingshan Formation are mainly distributed in the northeastern area of the ZG17-TZ72 well field, the area between the ZG17 well field and ZG22 well field, and the southwestern area of the ZG44-ZG53 well field.