Quantitative Modelling of Sedimentary Heterogeneities of the Tight-Sand Reservoirs in the Triassic Yanchang Formation, Ordos Basin, China

Abstract

The Triassic Yanchang Formation in the Ordos Basin is the best known tight oil petroleum system in China and accounts for the majority of the tight oil production in the country. The formation consists of three members with the Chang-7 organic-rich shale in the middle as the source and the Chang-6 and Chang-8 immediately above and below as reservoirs. Variabilities in production data suggest that sedimentary heterogeneities play a key role in controlling the sweet spot distribution. We applied SEDSIM, a three-dimensional stratigraphic forward modelling program, to simulate the sedimentary heterogeneities of the Yanchang Formation depositional system by considering key depositional processes and parameters affecting the lacustrine and deltaic deposition including tectonic subsidence, lake level fluctuation, sediment supply, and pre-existing topography. A nest-modelling approach was employed to model both basin scale (295×455 km) and reservoir scale (40×40 km) heterogeneities with spatial resolutions of 5 km and 1 km, respectively. The vertical (temporal) resolution is 50 kyr. The simulations show that the sedimentary heterogeneities were caused by the interplay of the initial basin configuration and topography, lake level fluctuations and sediment supply. Large-scale shallow-water deltas from multiple sources migrate toward the southeastly orientated lake basin in response to lake level fluctuations. The deltaic plain is dominated by distributary channel sandbodies, while the delta front is characterized by sheet sands as a result of shallow-water delta migration. The total amount of sediment discharge from the northeast and southwest sources accounts for 80% of the sediment load. The Chang-8 member has better reservoir properties and sandbody connectivities compared with that for the Chang-6 member. The sandbodies in the delta plain and delta front with porosity range of 7%-13% are the sweet spot targets for future exploration.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015