Characteristics and main controlling factors of the limy source rock gas reservoir in the first member of the Middle Permian Maokou Formation in the southern Sichuan and western Chongqing area: a case study of well DB 1

Abstract

The Middle Permian Maokou Formation in the Sichuan Basin is an important natural gas payzone. Recently, Zhejiang Oilfield Company of CNPC obtained industrial gas flow of vertical well test 42×103 m3/d and horizontal well (well DB 1H) test 556×103 m3/d in the limy source rock gas reservoir of eyelid and eyeball- shaped limestone in the first member of the Maokou Formation in well DB 1, Da'an exploration area, western Chongqing. A breakthrough has been made in the unconventional gas exploration of argillaceous limestone in the southern Sichuan and western Chongqing area, showing good exploration prospects in this area. The eyelid and eyeball-shaped limestone in the first member of the Maokou Formation in the Sichuan Basin has the characteristics of self-generation and self-storage, continuous and stable distribution and high thermal maturity. The limestone of central gentle slope facies shows no water sensitivity or acid sensitivity, and is dominated by carbonate brittle minerals, which is ideal for acid-fracturing development. It is of great significance to study the characteristics and main controlling factors of the organic rich limestone reservoir. Taking well DB 1 as an example, this paper carried out a study on the petrology, sedimentology and reservoir characteristics of the first member of the Maokou Formation, and it is found that, deposited on the central gentle slope with frequent water activity and abundant organism, the dark gray to black micrite with high organic matter content has good physical properties. In general, submember c has better physical properties than submember a. The development degree of macropores in eyelid-shaped limestone is higher than that in eyeball-shaped limestone, and the development degree of micropores is not much different. The main pore types of the first member of the Maokou Formation include dissolution pores, fractures, clay mineral apertures and organic pores. The pores in clay minerals (mainly talc) and organic matters are mainly distributed in the eyelid-shaped limestone. Depositional process and diagenesis are the main factors controlling the development of the eyelid and eyeball-shaped limestone reservoirs. The central gentle slope facies controls the development and distribution features of the organic-rich micrites. Diagenesis as well as fracture and dissolution effects improve the reservoir capacity of limestone, which is critical for hydrocarbon accumulation. Stable and high yield commercial gas flow was obtained in the vertical wells and horizontal wells in the first member of the Maokou Formation which has high organic matter content in well DB 1 block in the western Chongqing area, indicating that the accumulation and occurrence conditions of limestone source rock gas are good in the first member of the Maokou Formation. It has two kinds of accumulation modes with self-generation and self-storage dominance and short distance aggregation of local structural bands, and has the characteristics of continuous distribution of gas reservoir and large resource scale. The "continuous gas reservoir: in the limestone source rock in the first member of the Maokou Formation has great exploration potential, which will become an important exploration target with good potential in the Sichuan Basin.

Keywords

• limestone source rock gas
• reservoir characteristics
• first member of maokou formation
• middle permian
• southern sichuan and western chongqing area