Pore structure characteristics of non-marine shale in Ordos Basin, China
Authors: Fang Changliang , Amro Mohammed 
Published: March 2015

Unlike marine shale, the non-marine (lacustrine) shale deposits are with frequent sand and mud interbed, large gross thickness, interlayers development and etc. In order to study reservoir properties, adsorption, permeability, porosity, gas storage and migration of such shale, experimental study of porosity character with the core sample of Mesozoic Triassic Yanchang Formation shale in Ordos Basin of China and contrastive study of this non-marine shale porosity characters and marine shale porosity character in north America and China were integrated in this paper.

In experimental study, high pressure (up to 200 MPa) mercury porosimetry analysis determined the porosity, pore diameter and its size distribution, and specific surface area; different pressure permeability tests with nitrogen medium were taken to analysis the permeability with Klinkenberg correction; SEM images were observed to characterize the micro pore structures, and to describe the geometry, connectivity and fillings of pores. By combining all above experimental result, core structure features was comprehensive analyzed and gas-bearing properties of shale was evaluated.

In contrastive study, experimental and literature data of pores feature in non-marine shale and marine shale, including permeability, porosity, pore size, and SEM images, were compared to analyze the development and structure of shale porosity in different sedimentary environment.

Studies show that lacustrine shale has considerable gas storage capacity, and various types of porosities were observed on SEM images. Expectable, it could get considerable gas output. However, non-marine shale with extremely low permeability has adjacent porosity (accessible porosity 2.27%) to that of marine shale, which is attributed to the poor rate of pore connectivity in non-marine shale.

Introduction

As one of considerable oil-bearing basins in China, Ordos Basin, located in the western part of North China platform, is a large-scale superimposed intracratonic basin formed after multi-periodical tectonic activities. It is generally believed that shallow deposit of sedimentary strata in China is continental sediments and deep deposit is marine sediments, which is opposite to the situation in Middle East, United States, Russia and Europe. In the Late Paleozoic-Middle Triassic period, Ordos Basin is in the process of transforming from marine and continental alternately sedimentary environment to continental sedimentary environment (Wu et al., 1993). Mesozoic Triassic Yanchang Formation is a terrigenous clastic rock sedimentary system mainly with inland lakes - delta deposition. On the area, Yanchang Formation generally is divided into 10 lithological combination members. The core samples having been tested are from Chang 7 member of Mesozoic Yanchang Formation. This section of Yanchang Formation is main oil source rock of lacustrine deposit in Ordos Basin, and the lithology of Chang 7 member is mainly dark mudstone, oil shale with thin layer of powder, fine sandstone, meanwhile pyrite particles are produced.

Unlike marine shale, the non-marine (lacustrine) shale deposits are with frequent sand and mud interbed, large gross thickness, interlayers development and etc. In order to study reservoir properties, adsorption, permeability, porosity, gas storage and migration of such shale, experimental study of porosity character with the core sample from Chang 7 member of Mesozoic Triassic Yanchang Formation shale in Ordos Basin of China and contrastive study of this non-marine shale porosity characters and marine shale porosity character in north America and China were integrated in this paper.