A New Structure and Fault-Controlled Meteoric Karstification: A Case Study of Ordovician Carbonate Reservoir at Yingmai Area, Tarim Basin, Northwest China

Qiao, Zhanfeng ¹; Shen, Anjiang ¹; Shou, Jianfeng ¹; Ni, Xinfeng ¹; Zheng, Jianfeng ¹
(1) Hangzhou Institute of Petroleum Geology, Petrochina, Hangzhou, China.

A new meteoric karstification controlled by structure and fault, which is discriminated from the conventional phreatic water table-controlled karstification, has been recognized by means of the observation of 12-wells cores and analysis of geochemical data.

Structure Yingmai 2# of the north of Tarim Basin, northwest China, formed during early Hysiannian. With the significant uplift of he north of Tarim Basin under the background of compressing and shearing stress, Structure Yingmai 2# took on the cove with the two orients of strick -slip faults on the top. The target layer, lower-middle Ordovician pure carbonate, is covered by upper Ordovician and Silurian mudstone with the thickness of 700~1000 meters. With the uplift of structure and development of faults, fractures are prone to occur at the top of the carbonate. The high topography induced that the meteoric water took the high hydraulic potential power so as to flow along the faults into the carbonate strata, resulting in the change of temperature-pressure system, in addition that the fractures occurred at the top of structure elevated the water-rock ratio significantly, which are in favor of the karstification.

The 18O of the calcite relative to the hydrothermal fluid is depleted with the range of -10~-15‰PDB, and the 13C value is elevated. The horizontal fracture formed during the structure uplifted, the fillings of which is mainly strata fluid with the high temperature, so that the 18O value is depleted significantly less than -15‰PDB, and the 13C value is also depleted. However, the calcites filling in the high-angle fractures and fissures have the less depleted 18O value in a range of -5~-15‰PDB, as a subsequence of the cooling of the meteoric water flowed down along the faults.

The 87Sr/86Sr ratios of the calcites filling in the fractures and caves vary ranging from 0.7087 to 0.710 appreciably more than those relative to the hydrothermal fluid and the corresponding value in temporal seawater, which has been considered as the result of the meteoric water flowing through the siliciclastic strata with higher composition of crust-source 87Sr decayed from radioactive 87Rb.

The calcites filled in fractures and caves are mostly dull or non-doluminescence that is low concentrations of Fe2+ and Mn2+, whereas, the calcites related to hydrothermal fluid are light orange doluminescence with relatively high concentrations of Mn2+ and Fe2+

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.