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Abstract
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.
中国西北塔里木盆地中部的塔中地区具有丰富的油气资源和复杂的地质条件，主要是在下奥陶统碳酸盐岩储层。尽管经过多年的探索和发展，油气成藏机制和其关键控制因素仍不清楚。由于缺乏这方面的了解，使得对中下奥陶统鹰山组进行油气勘探受到限制。在此，检验了对地质条件及储层分布特征的分析、积累机制和关键因素，以预示目标勘探区。我们的结果表明，经历了四个积累阶段的油气，有多种来源，包括塔中地区及以外的寒武系 - 下奥陶统和中上奥陶统源岩。碳氢化合物沿东北部和西北部的断层组10个交叉口迁移到风化壳岩溶储层，然后主要在西北 - 东南方向横向迁移。鹰山组碳酸盐岩储层典型地由岩性来定义，主要由不同的毛细力来控制。油气在鹰山组的成藏和分布是由极好的储蓄层和油气补充能来控制。优秀的储蓄层的物理特性控制着油气特征和油气成藏临界点： 这些储层具有临界点为最小的孔隙度和渗透率，约1.8％和0.1mD，以及临界点为最大毛细力，大约2.5MPa。油气充入能控制油气富集范围：富集于注入点附近的油气和油气充入距离临界点20公里 。最佳的储层和油气充入能耦合指数（RSI）控制油气成藏和分布。当RSI值<0.5时成藏不会发生，但指数越高，越有利成藏。耦合指数用于预测鹰山组的岩石藏层主要分布在ZG17-TZ72井场的东北部、ZG17井场和ZG22井场之间的区域以及ZG44-ZG53井场的西南部。
ACKNOWLEDGEMENTS
This work was supported by the National Basic Research Program of China (973 Program) (No. 2011CB201100), Chinese Ministry of Education. We thank the Tarim Oilfield Exploration and Development Research Institute for providing background geological data and permission to publish the results.
Disclosure statement
No potential conflict of interest was reported by the authors.
Editorial handling: Keyu Liu