The Contrastive Research of Reservoir Characteristics between the Fifth Member of Xujiahe Formation in Western Sichuan Depression and Typical Shale in USA

Lei Chen, Zhenxue Jiang, Jinyan Xing, Dongxia Chen, and Weitao Chen
Unconventional Natural Gas Institute, China University of Petroleum, Beijing, China

The research of shale gas reservoir characteristics is the basis of evaluation for shale gas accumulation conditions and reservoir fracturing transformation.

Based on the analysis of single-well reservoir geology and integrated with other data of previous research results, drillings and outcrops, we contrasted the fifth member of Xujiahe formation shale in western Sichuan depression with the five major shale gas reservoirs in USA by delineating its organic geochemical characteristics, mineral compositions, physical properties and mechanical properties. It is initially confirmed that the fifth member of Xujiahe formation shale in the study area is similar to the main producing gas shale in the United States in terms of four significant reservoir features. It is proved to be economic recoverable value of shale gas in the study area, so the article can provide some guidances for the exploration and development of continental shale gas in western Sichuan depression.

1. Organic geochemical characteristic

Shale with organic carbon content above 0.5%, is generally regarded as source rock with hydrocarbon generation potential. The organic matter content varies as lithology changing and shale with rich clay is relatively higher. The total organic carbon content of the five major gas-bearing shales in the United States usually ranges from 1.5% to 20% and the total organic carbon content in the study area ranges from 0.39% to 16.33%. Shale of different kerogen types can generate a large amount of gas, research suggests that the types of kerogen do not affect the gas production of source rock, the total organic content and the thermal maturity are two key factors which can determine the ability for gas generating. The organic matter types of gas-bearing shale in the United States are predominantly type I and type II, part of them are type III, only the Lewis Shale in the San Juan Basin contains primarily Type III, however, the fifth member of Xujiahe formation shale in the study area is basically the type III kerogen. The organic matter maturity determines not only the generation way of natural gas, but also the components of the gas. The maturity of the five major shales in the United States ranges from 0.4% to 2.1%, the maturity of the study area ranges from 1.02% to 1.68%, basically entered high/over thermal maturity phases.

2. Mineral composition

The mineral compositions of shale are complicated, in addition to kaolinite, smectite, illite and other clay minerals, but also mixed with quartz, feldspar, mica and other detrital minerals and authigenic minerals. One notable feature of the United States shale is with high silica content (commonly more than 50% and some can reach up to 75%), most of them is clay fraction and often occur in the form of laminal. The content of brittle minerals in the study area is high (up to 79%) and the content of clay minerals is slightly less than United States.

3. Physical property

In the conventional reservoir analysis, porosity and permeability are the two most important parameters of reservoir characteristics, which also apply for shale gas reservoirs. The five major gas-bearing shale in the United States has porosities ranging from 3% to 14%, permeability are generally less than 0.1mD. But the sealed fractures can reactivate through hydraulic fracturing stimulation treatments, so the permeability of shale will be enhanced efficiency. The data of the fifth member of Xujiahe formation shale in the study area are rare, limited physical property data indicate that shale physical properties in the study area are quite poor and belong to the extremely tight shale reservoirs.

4. Rock mechanical property

Abundant silica content makes shale with a characteristic of strong brittleness, the shale formations are generally rigid and characterized by relatively high elastic modulus and low Poisson's ratio, which can easily form natural fractures and induced fractures under the action of external force. In general, the brittle shale has a higher gas production. The five major gas-bearing shale reservoirs in the United States have high silica content and strong brittleness, and easy to be fractured. The fifth member of Xujiahe formation shale in the study area is also with a high brittleness, in which Xinchang 28 well has been successfully fractured with gas production.

In summary, comparing with the organic geochemistry characteristics of the successful produced gas-bearing shale in the United States, although the organic matter type of the fifth member of Xujiahe formation shale in the study area is type III kerogen, it is good for gas generating and have a better capacity of hydrocarbon generation, furthermore, some geochemical indicators such as total organic carbon content and maturity are higher than most of the gas-bearing shale in the United States, it is well known that organic carbon content and maturity are the most important factors of accumulation conditions. The brittle minerals content of the study area is relatively higher than the United States and easy to be the later reconstruction, rock mechanical properties also show low Poisson's ratio and high elastic modulus, which is similar to gas-bearing shale of the United States. The shale physical properties of the study area are quite poor, although there are fractures developed in the shale, most have been filled with carbonate minerals, and part of matrix porosity are partially blocked with calcite.

The contrastive research of reservoir characteristics between the fifth member of Xujiahe formation in western Sichuan depression and typical shale in the United States indicates that the shale in the study area is of good reservoir conditions for shale-gas exploration and development, so the study area can be regarded as the favorable zones of Sichuan basin for shale gas exploration.

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