地质统计学反演应用

Geostatistical Inversion in Thin Limestone Reservoir of Low Porosity and Low Permeability:

The Application in Da ’anzhai member in the eastern Sichuan Basin

Li Zhongping, Deng Ping, Yu Xiaona (Jianghan Oil Field, Sinopec) He Yuting*, He Xinwei, Zhang Wanlong (Jason, Beijing office)

Summary

The prediction of thin limestone reservoir is an international problem in seismic exploration. The limestone reservoir of Da ’anzhai member in the eastern Sichuan Basin features super tightness, strong heterogeneity and thin layers. Based on post-stack deterministic Constrained Sparse Spike Inversion, this paper applies geostatistical inversion to predict the thin limestone reservoir of Da ’anzhai member. The geostatistical inversion method can effectively integrate geological, well logging and seismic data, taking advantage of the higher vertical resolution of well logging data and higher lateral resolution of seismic data, describing the subtle changes in the reservoir more accurately, thus it works well in identifying thin limestone reservoir with strong heterogeneity. Furthermore, the formation porosity is achieved by co-simulation, and the predicted results match well drilling result well.

Introduction

The study area is located in the eastern Sichuan Basin, where the thin biogenic limestone of Da ’anzhai member of Ziliujing formation in Lower Jurassic is one of the major reservoirs. The reservoir type is lithological one controlled by reservoirs. The Da ’anzhai member in this study area is a suite of lacustrine facies deposits, with gentle structures. There are three sub-members (Da 1, Da 2 and Da 3) in Da ’anzhai member. The limestone in Da 1 sub-member is the thickest and the regional major reservoir. Da 2 sub-member is almost all black shale, the major source rock of Da ’anzhai member. Da 3 sub-member is thin limestone interbedded with dark shale, but it is thinner.

This major target layer in this study is Da 1 sub-member, whose depositional environment is lakeshore-shallow lake, with the major lithology of shelly limestone depositing in high energy environment, interbedded with a little dark shale. Its thickness about 30m, it is buried at 2100-2200m deep. The porosity of limestone matrix is generally less than 1%, and its permeability is usually less than 0.1md. The analysis results of 48 core samples from this study area show that Da 1 sub-member has the porosity of 0.2%~4%, and permeability between 0.07~5md, most samples’ porosity are less than 2%, and permeability less than 1md, belonging to extremely low porosity and low permeability rocks. However, some samples have the higher porosity and permeability. The reservoir has stronger heterogeneity

(Fig. 1). The reservoir thickness of single layer from well logging interpretation is 1~5m, most are about 2~3m, with features of thin reservoir (Fig. 2). The discovered reservoirs in Da 1 sub-member in this area and adjacent areas have no apparent edge water and bottom water, without oil and water recognition issue in reservoir prediction. Whether the reservoir is good or bad is mainly affected by porosity. About half a century ago, commercial oil and gas flows had been found in Da 1 sub-member of several wells in adjacent areas. In the past few years, commercial oil and gas flows had also been found in Da 1 sub-member in two wells in this study area, indicating promising oil and gas prospect. But because of the reservoir is tight, strongly

heterogeneous and thin, its prediction is quite challenging.

Figure 1, Distribution graphs of porosity(a) and permeability(b) of Da 1 sub-member, data from core analysis

Figure 2, Lithology interpretation profile of well logging data of

Da 1 sub-member in Well A.(Green: mudstone, Pink: porous limestone, Blue: tight limestone)

D o w n l o a d e d 09/16/13 t o 106.37.165.251. R e d i s t r i b u t i o n s u b j e c t t o S

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