An examination of the fluid inclusions of the well RP3-1 at the Halahatang Sag in Tarim Basin, northwest China: Implications for hydrocarbon charging time and fluid evolution

The well RP3-1 lies in the Tarim Basin, which is located in the southern Xinjiang Uygur Autonomous Region in northwest China. This region is one of the world's largest frontier basins. Drilling data that we collected on the well RP3-1 reveal a 2398-7066 m thick sedimentary sequence, containing Ordovician to Tertiary strata. The Yijianfang (O2yj) formation of the Middle Ordovician sequence is one of the most important reservoir stratums. The O2yj formation consists mainly of bioclast and intraclast limestones. Two different fluid types are identified by the associations they have with veinlets, which are colors caused by petroleum inclusions under UV light. Early-stage petroleum fluids are characterized by a blue-white color under UV-epifluorescence microscopy, and early-stage aqueous fluid inclusions are mainly homogenized between 54 and 102 °C, with salinities ranging from 12.0 to 16.7 wt% NaCl equiv. Late-stage petroleum fluids are characterized by a yellow color under UV-epifluorescence microscopy, and late-stage, aqueous-fluid inclusions are mainly homogenized between 85 and 135 °C, with salinities ranging from 2.0 to 8.0 wt% NaCl equiv. In order to obtain accurate measurements of both the trapped temperature (Ttrap) and pressure (Ptrap) of these fluids, we use 10 pairs of petroleum inclusions and coeval aqueous fluid inclusions to measure the intersection of a number of isochors. The Ttrap of early-stage fluids we surveyed were between 82 and 97 °C, with the Ptrap ranging from 97 to 259 bars. In contrast, the Ttrap of late-stage fluids were between 141 and 170 °C, with Ptrap values ranging from 430 to 608 bars. When combined with the regional buried history, the early-stage fluids charged from 422 Ma to 412 Ma BP, and the late-stage fluids charged from 20 Ma to 6 Ma BP.