Forward Depositional Modelling of the Triassic Chang-8 Tight Sandstone Reservoir in the Ordos Basin, China

Xiu Huang^1,2 and Keyu Liu^1,2,3
1Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China
²CNPC Key Lab. for Basin Structure and Hydrocarbon Accumulation, Beijing, China
³CSIRO Earth Science and Resource Engineering, Bentley, WA, Australia

Tight sandstone reservoirs are generally characterized by fine sedimentary grains, relatively high organic contents, strong heterogeneity, and low porosity and permeability. Their formation often relates to interactions between tectonics, palaeogeomorpholoty, sediment provenance, depositional process and environment. Post-depositional processes such as tectonic stress, compaction, and fluid evolution, all act upon the initial sedimentary framework and heterogeneity to produce the final heterogeneous tight reservoirs at various scales. Several studies have emphasized that depositional facies, combined with diagenetic and stress histories exert a fundamental control on the evolution of fluid flow properties in unconventional reservoirs, with respect to both matrix and fracture characteristics. However, the quality of most tight reservoirs depends primarily on the original environmental variations within depositional systems (Bloch and McGowen, 1994). Therefore accurate understanding of the original depositional environment is essential for reservoir characterization in tight sandstone reservoir investigation.

The Ordos Basin, located in the western part of North China, is an intracratonic basin. During the deposition of the Yanchang Formation in the Upper Triassic period, the basin was a shallow depression characterized by an extremely low gentle morphology and a stable interior tectonic setting. It was filled by over 1000-1300 m thick fluvial-deltaic-lacustrine sequences, the Yanchang Formation, which is one of the most important petroleum reservoir sequences in China. Due to the presence of an effective cap rock of the Chang-7 member, which is dominated by strongly compacted mudstones covering extensively over the basin, the underlying Chang-8 member is an important reservoir interval in the Ordos Basin and has been intensively explored. Although many studies provide useful depositional models for the Chang-8 member, these models cannot adequately capture the heterogeneity of the complex depositional system. They are thus lack of good predictability of the potential reservoirs in the basin due primarily to low resolution of seismic data, limited numbers of wells and outcrop exposures. In order to address these problems, we applied a stratigraphic forward modelling program, Sedsim, to model the depositional system of the Chang-8 member and its facies distribution in fine scales and to predict the potential reservoirs within the basin. By invoking the Naviere-Stokes equations, Eulerian and Lagrangian equations (Tetzlaff and Harbaugh, 1989), Sedsim is capable of directly simulating the depositional processes and their products during a basin evolution with limited information. The simulation outputs can be used to extrapolate sedimentary facies in the undrilled area away from known wells (Griffiths et al, 2001).

We developed a three dimensional depositional model using Sedsim for the Chang-8 member over an area of 295 km x 495 km with a horizontal resolution of 5 km and over a duration of 2.8 Ma between 220.3 Ma and 217.5 Ma with a vertical sampling resolution of 50 ka. The environmental conditions considered in constructing the model include the initial bathymetry, base level changes, tectonic movement, sediment provenance, and controls on primary and post-depositional porosities. The final simulation quite well matches the known depositional thickness and facies in some key locations and thus provides a realistic model for predicting sedimentary facies distribution and reservoir heterogeneity in the entire basin over the simulated interval. It is shown that the bathymetric morphology plays an important role in governing the deltaic migration. For instance, river-deltas from multiple sources migrate toward the lake centre located in the southeast of basin. The thickness of deltas decreases away from the sources. The delta plain is dominated by distributary channel sand-bodies, while the delta front is characterized by sheet sands, which are common on extremely low slope. The total amount of sediment discharge from the northeast and southwest sources accounts for 80%, indicating that they are the two main sources supplying to the basin. In addition, the simulation shows that areas with the favourable permeability (0.3 mD to 1 mD) for sustainable development as proposed by Yao et al. (2013) mainly correspond to the sand-rich delta-plain and delta-front. Those sandbodies have good porosities with ranges from 7%-13% and are potential exploration targets. The Sedsim simulation successfully reproduced the geologic record of the Chang-8 member (e.g. thickness, shape, grain size and location) with accuracy, and also provide insight for locating favourable reservoir exploration regions (both delta-front and delta-plain), which were previously not considered.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013