Natural gas characteristics, fluid evolution, and gas charging time of the Ordovician reservoirs in the Shuntuoguole region, Tarim Basin, NW China

The Shuntuoguole region is situated at the center of the Tarim Basin, in NW China. It is bordered by the Tazhong Uplift to the west and the Manjiaer Depression to the east. Data obtained on samples collected from exploration wells highlight the large potential for gas discoveries in this region. Component and stable carbon isotopic analyses have been performed on gas samples from 5 wells in the Shuntuoguole region. The dryness coefficients of gas samples vary from 0.95 to 1.00, indicating that the gas samples belong to overmatured dry gas. The carbon isotopic values (delta C-13) of all gas samples are heavier than -40 parts per thousand, compared to the typical gas samples with different sources in the Tarim Basin, revealing that the gas in the Shuntuoguole region originated from source rocks in the Cambrian. The SN-1 exploitation well was selected to undertake fluid inclusion studies revealing the mechanisms of natural gas charging and fluid evolution. Data from the SN-1 well drill core indicate a 1,700- to 7,050-m-thick sedimentary sequence containing Ordovician to Tertiary strata. The Middle Ordovician Yijianfang Formation (Fm) constitutes one of the most important gas reservoirs. The Yijianfang Fm consists principally of limestone containing widely distributed sparry calcite. Numerous fluid inclusions are observed in calcite, and 4 types of fluid inclusions are recognized based on the observed phases and components. These are pure CH4 (P-type), CH4-bearing (C-type), aqueous (W-type), and solid-bearing (S-type) inclusions. The trapping temperatures and pressures, determined through the intersection of isochores from coeval fluid inclusions, reach 183 degrees C to 216 degrees C and 1,914 bars to 2,196 bars, respectively. When taking into account the regional burial history, we find the natural gas charged during the Neogene interval, from 20Ma to 3Ma.