158 / 2019-06-02 16:03:54

Characteristics and genesis of sucrosic dolomite in Middle Permian Qixia Formation，Northwest Sichuan Basin：A case study from Shangsi section

Dolomite genesis,Seepage reflux dolomitization,Hydrothermal dolomization,Karst,Middle Permian Qixia Formation,Northwest Sichuan Basin

A major breakthrough has been made in the gas exploration on sucrosic dolomite reservoir of the middle Permian Qixia Formation in the northwest Sichuan Basin. However, recent cores show that the development horizon and scale of the sucrosic dolomite is of great variability. Therefore, the key to the distribution prediction of this type of dolomite reservoir is to figure the genetic mechanism out. Taking the Shangsi section as an example, and based on the data from petrologic features, carbon and oxygen isotopes and EPMA(electron probe micro analyzer), four characteristics of the sucrosic dolomite were summarized, including: 1) The dolomite can be divided into massive dolomite, developing well in the middle of Qixia Formation member 2, leopard dolomite from dolomitic limestone, developing in the lower part of member 1, lower part and upper part of member 2; 2) Massive dolomite with medium-coarse-crystalline structure is characterized by saccharoidal texture and red luminous under CL(Cathode Luminescence), with Mg/Ca values 0.68, δ13C and δ18O average values 2.42‰ and -6.18‰ respectively; 3) Leopard dolomite developing in the lower part of Qixia Formation member 1, with powder-fine-crystalline structure is characterized by micro-sucrose texture and non-luminous under CL, with Mg/Ca values 0.54, δ13C and δ18O average values 2.42‰ and -6.18‰ respectively; 4) Leopard dolomites developing in the member 2 with saccharoidal texture has different characteristics. The upper part dolomite with medium-fine-crystalline structure shows a non-luminous core inside and a red luminous rim outside under CL, while the lower part dolomite with medium-coarse-crystalline structure shows dark red luminous under CL. Their Mg/Ca, δ13C and δ18O average values are 0.59 and 0.68, 2.16‰ and-6.50‰, 3.32‰ and -3.75‰ respectively. Based on the results from filed work and indoor materials arrangement, it is believed that in the lower part of Qixia Formation, there are two syngenetic exposure faces, under which there was leopard porphyritic karst system. A eogenetic exposure face has been identified on the top of Qixia Formation with clear vertical zonation of karst below. Most of the karst dominant channels have been dolomitized. Both types of dolomites were formed by seepage reflux dolomitization and hydrothermal dolomitization. Due to the vertical zonation and influence degree of karst, there is differences between the two phases dolomitization. This study can provide not only a basis for studying the dolomite genesis but also a reference to gas exploration deployment of Qixia Formation in this area.