Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

Wen-Tao Zhao,Gui-Ting Hou

doi:10.1007/s12182-016-0141-2

Abstract

It is important to predict the fracture distribution in the tight reservoirs of the Ordos Basin because fracturing is very crucial for the reconstruction of the low-permeability reservoirs. Three-dimensional finite element models are used to predict the fracture orientation and distribution of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on the distribution of sand bodies in the Chang 71 and 72 members, and the different forces that have been exerted along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Cenozoic are NW–EW and NNE–ENE, respectively. In this paper, the two-factor method is applied to analyze the distribution of fracture density. The distribution maps of predicted fracture density in the Chang 71 and 72 members are obtained, indicating that the tectonic movement in the Late Mesozoic has a greater influence on the fracture development than that in the Cenozoic. The average fracture densities in the Chang 71 and 72 members are similar, but there are differences in their distributions. Compared with other geological elements, the lithology and the layer thickness are the primary factors that control the stress distribution in the study area, which further determine the fracture distribution in the stable Ordos Basin. The predicted fracture density and the two-factor method can be utilized to guide future exploration in the tight-sand reservoirs.

Highlights

Unconventional oil and gas resources, such as tight gas, tight oil and shale oil, have been successfully developed commercially in the USA, Canada, Australia and some other countries
Because the orientation and the distribution of structural fractures are the key elements in fracture prediction, the fracture orientation and the estimated density have been calculated with the finite element modeling and will be compared with the observed data in outcrops and cores
Since reliable numerical models are the basis of further study on the fracture prediction in the Longdong area, it is necessary to verify the correctness of the two models proposed in this paper, including the Late Mesozoic and the Cenozoic ones, by comparing the results of finite element modeling with earlier published data

Summary

Introduction

Unconventional oil and gas resources, such as tight gas, tight oil and shale oil, have been successfully developed commercially in the USA, Canada, Australia and some other countries. In the Ordos Basin, the tight-oil reservoirs in the Triassic Yanchang Formation have become a major target of petroleum exploration and development in recent years (e.g., Guo et al 2012; Yao et al 2013). Since tight-oil reservoirs in the Ordos Basin are of lowpermeability (\2 9 10-3 lm2) and low-porosity (\10%) overall, fracturing is crucial for the reservoir reconstruction, even though the reservoirs are formed with complicated mechanisms (e.g., Yao et al 2013; Ezulike and Dehghanpour 2014). It is important to predict the natural fracture distribution in reservoirs, including

Results

Discussion

Conclusion