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Tom Kirkman

Reservoir characteristics and logging evaluation of gas−bearing mudstone in the south of North China Plain

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Something a bit different from Nature dot com popped up in my feed today. 

A compressed PDF of the entire 11 page article is attached at the bottom of this comment.  The full 7 MB uncompressed PDF is available in the link:

 

Reservoir characteristics and logging evaluation of gas−bearing mudstone in the south of North China Plain

Abstract

Mudstone is very similar to shale except it lacks sheet bedding. Shale gas is widely concerned and successfully exploited commercially in the world, while gas-bearing mudstone is rarely paid attention. To evaluate the reservoir characteristics and exploitation potential of gas-bearing mudstone, a total of 127 mudstone samples from the Shanxi formation were tested by X-ray diffraction (XRD), scanning electron microscope (SEM), gas content, etc., and the qualitative identification and quantitative evaluation of gas-bearing mudstone reservoirs were performed on four wells using the logging curve overlay method and reservoir parameter calculation equations. The results showed that:

(1) the average total gas content of core measurement is 1.81 m3/t, and the total content of brittle minerals is 44.2%, which confirms that mudstones can also have good gas content and fracturing performance;

(2) logging evaluation the average thickness of gas-bearing mudstone is 55.7 m, the average total gas content is 1.6 m3/t, and the average brittleness index is 38.1%, which indicates that the mudstone of Shanxi formation in the study area is generally gas-bearing and widely distributed.

All the results reveal that gas-bearing mudstone with block bedding has the same exploitation potential as shale with sheet bedding,which deserves more attention.

Introduction

Mudstone and shale are very similar. The difference is that the mudstone lacking the laminations of shale, which easily forms a block shape when it is broken, while shale has bedding structure, which splits easily into thin flat layers. Although both are commonly referred to as shale in a broad sense, the source rock is mainly composed of mudstone and siltstone. In order to comply with the actual characteristics of the formation in the study area and avoid confusion in concept, the concept of mudstone in this paper refers to relatively homogeneous mudstone with block bedding, including silt mudstone, and the shale refers to strong heterogeneous mudstone with sheet bedding, including siltstone.

There has been a lot of research on shale gas worldwide. The famous shale gas production areas include the Barnett, Fayetteville, Haynesville and Marcellus in the United States, the Montney and Horn River in Canada, and the Fuling and Changning in China, etc. It is a fossil energy stored in unconventional reservoirs and has great exploration prospects. For shale gas reservoirs, the shale itself is both a source rock and a reservoir. Shale gas is mainly stored in two forms, one is in the free state in the cracks and intergranular pores, the other is adsorbed on the surface of kerogen and clay particles. In addition, there may be very little shale gas dissolved in the bound water, kerogen and bitumen. Gas-bearing mudstones and shale have similar reservoir characteristics, and many shale gas production areas in China are often interbedded with mudstone and shale rather than single lithology of shale. Therefore, many chinese scholars have studied them as integration and defined them as mud-shale or mudstone-shale reservoirs.

There are mud-shale reservoirs in the Shanxi and Taiyuan formations in the south of the North China Plain. These reservoirs have a stable gas production of more than 3000 m3/d in some wells, which indicating good prospects for mud shale gas exploration and development. However, according to the data of cores and drilling cuttings, the mudstones of Shanxi formation of four wells A1, A2, A3 and A4 in the study area of this paper have no bedding. For this mudstone formation, it has been evaluated as a cap rock and a source rock for a long time, and it is rarely considered as a reservoir. In order to find out whether mudstone formation has the same mining potential as shale or mud-shale formation, this paper focuses on the mudstone of the Shanxi formation of the four wells A1, A2, A3, and A4 in the southern North China Plain. Firstly, a total of 127 core samples of Shanxi formation mudstone in well A2 were tested by X-ray diffraction (XRD), electron microscopy (SEM), total organic carbon content (TOC), vitrinite reflection (RO) and gas content respectively, and their mineral characteristics, reservoir space types, physical properties, hydrocarbon generation capacity and gas content were summarized. Then, based on the well logging data, the well logging response characteristics of mudstone formations with different gas contents were analyzed. Finally, qualitative identification of gas-bearing mudstone reservoirs and quantitative evaluation of mudstone formation sedimentary environment, gas content, porosity, permeability, and brittleness index were performed using well logging methods.

Geological Setting

After the Indosinian movement, the mainland of China was dominated by intracontinental deformation. Due to the collision and compression of the Qinling-Dabie orogenic belt, since the Late Triassic, the depression began to appear in the north of South China, resulting in the fault between wells A3 and A4.  ...

 

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Compressed PDF attached:

Reservoir characteristics and logging evaluation of gas−bearing mudstone in the south of North China Plain (compressed copy).pdf

 

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