Accurate identification of overflow fluid types facilitates timely and effective handling of onsite overflow accidents. Research into identifying the type of overflow fluid is limited, and there are only simple calculation models that do not consider enough effects; additionally, accuracy needs to be improved and the identification method is not perfect. If there is no drilling data, it is impossible to identify the overflow fluid. Therefore, this paper modifies the density calculation model of overflow fluid by considering the influence of the temperature, pressure field, and two-phase flow model, making the calculation result more accurate and universal, and puts forward a comprehensive method for auxiliary identification based on gas logging interpretation. This paper uses the gas state equation to verify the accuracy of the overflow density model; after verification using data from more than 20 overflowing wells, the method was found to be practical and had an accuracy rate of more than 90%. Thus, this study and the proposed method can provide guidance for dealing with overflow accidents in the field and any follow-up research.

Authigenic minerals from Chang 8 sandstones can preserve a precise paleotemperature record. Petrographic observations and geochemical characteristics provide a new insight into the formation mechanism of chlorite and calcite and resolve the problems of mobile elements between them. Chlorite is the most widely distributed mineral in the study area. In contrast, kaolinite is absent in some wells and intervals, which has implications for complete chloritization in the sandstones with abundant metamorphic and volcanic rock fragments due to the mixed provenances. Two stages of calcite cement can be identified: (i) early mesogenic, blocky calcite cement (GPI-Ca) precipitated from 84 to 107 °C in the oil-bearing sandstones (δ13CVPDB from −4.8‰ to −17.5‰; δ18OVPDB from −14.5‰ to −17.1‰); and (ii) late mesogenic, poikilotopic calcite cement (GPII-Ca) precipitated from 122 to 137 °C along the sandstone and mudstone (δ13CVPDB from −3.6‰ to −6.8‰; δ18OVPDB from −19.7‰ to −21.6‰). Grain-coating chlorite appeared to form from berthierine precursors from 70 to 110 °C, corresponding to the precipitation temperature of GPI-Ca. Pore-filling chlorite from kaolinite and illite formed at a temperature higher than 125 °C, corresponding to the precipitation temperature of GPII-Ca. GPI-Ca is characterized by nonferroan cement because the isomorphism contributes little at low temperatures and early-stage chloritization suppresses Fe into calcite. GPII-Ca without abundant metamorphic and volcanic rock fragments shows that ferrocalcite coexists with nonferroan calcite, but ferrocalcite does not exist in the sandstones with mafic minerals. Therefore, the chloritization of kaolinite is suitable for more Fe into the octahedral sites of chlorite in the diagenetic system rather than into calcite cement. The results of this study demonstrate the formation mechanism between chlorite and calcite through fluid-rock interactions. Better insights of the diagenetic sequence are significant to reconstruct the pore evolution history.