Geochemical properties of formation water reflect the sedimentary environments and the sealing conditions of the formation, which are of great significance for oil and gas exploration. In this research, the formation waters in the deep Permian Fengcheng Formation in the Mahu Depression were studied by analysis of chemical composition, stable isotopes, and the homogenization temperatures and salinities of fluid inclusions. The results show that the formation water is NaHCO₃-type with high salinity, high HCO^3-, low concentrations of Ca²⁺ and Mg²⁺ and high pH. The overall sealing of the formation is well maintained. Na₂SO₄-type water was found in some places with poor preservation of oil and gas. The fluid geochemistry and alkaline mineral distribution in the strata, which are dominated by deep fine-grain mixed deposits, indicate that the main body of the Fengcheng Formation was deposited in an alkaline lake sedimentary environment. The isotopic signatures are characterized by high δ¹³C, negative δ¹⁸O, and light strontium and heavy boron isotopes in the strata near the deep fault zone, which indicate that the Fengcheng formation may have been affected by deep hydrothermal fluids. From the first member to the third member of the formation, the salinity first increases and then decreases. This low-high-low trend corresponds to the initial stage of alkaline lake development followed by strong alkali formation and a final stage of water retreat. The coincidence between the spatial distribution of alkaline minerals, high-salinity formation water and source rocks indicates that the alkaline lake environment can form high-quality source rocks with good petroleum generation capacity.

The Shanxi Formation Shan-2 Lower Sub-member transitional shale in the eastern Ordos Basin is characterized by high total organic carbon value, wide distribution, and large single-layer/cumulative thickness; it is the key target interval for transitional shale gas exploration. Based on drilling, logging, 3D seismic, and natural energy spectrum data, this study discusses the spatial distribution and controlling factors of complex lithology in the transitional shale stratum. Using the multi-lithological eigenvalue method, the lithologic differential distribution characteristics of the Shan-2 Lower Sub-member were identified through seismic attribute analysis and post-stack seismic parameter inversion. On this basis, the controlling factors of lithology spatial difference distribution are revealed by paleogeomorphology restoration, the sedimentary environmental indicator characteristics of shale, and seismic facies division. The results indicate that the lithology distribution of transitional shale strata in the Shan-2 Lower Sub-member is significantly different, and the geomorphological pattern of interjacent depressions and highlands lays a foundation for the formation of the sedimentary system. The environmental and energy differences of paleo-sedimentary water form a sedimentary environment with frequent facies transitions, which controls the distribution of sediments. Th/U and Th/K radios, which indicate redox conditions and sedimentary water energy during the early sedimentary stage of the Shan-2 Lower Sub-member, show that the geomorphic uplift area is characterized by a high-energy transitional environment of mainly developed sandstone and silty mudstone, while the depression is relatively low-energy under a brackish water reduction environment, mainly composed of both dark and carbonaceous shale. During the late sedimentary stage of the Shan-2 Lower Sub-member, when geomorphic control was weakened, it presented a shallow-water, high-energy, and oxidized transitional environment, with a lithology dominated by the complex interbedding of fine-grained sandstone, silty shale, shale, carbonaceous shale, and coal seam. Therefore, it is inferred that the Shan-2 Lower Sub-member may have developed as a barrier island-lagoon sedimentary model. The early depositional period was dominated by barrier island-lagoon-tidal flat depositional combination, thence the sedimentary environment gradually evolved into a tidal flat-swamp environment.

Multicomponent seismic exploration provides more valuable information for the prediction of underground structures, lithology, fluids, and fractures. Most studies on lithology estimation and fluid description using PP- and PS-wave seismic data mainly focus on relatively shallow targets with high porosity. Focusing on deep carbonate (dolomite) reservoirs with low porosity (generally less than 5%), we first analyze the characteristics of the seismic response difference between PP and PS waves, and then we combine logging data and seismic forward modeling to implement the identification of fluids using the amplitude difference between PP and PS waves. Data sets are acquired over grain beach dolomite reservoirs in the Cambrian Longwangmiao Formation, the central Sichuan Basin, Western China. The reservoirs are filled with different types of fluids (e.g., gas, water, bitumen, etc.). Based on comparisons between seismic responses of PP and PS waves, we observe that, at the location of the gas-bearing layer, PP-wave amplitude exhibits strong peaks and PS-wave amplitude finds relatively weak peaks, which is different from that obtained for the water-saturated layer. However, at the location of bitumen-filled reservoirs, PP- and PS-wave amplitudes exhibit strong peaks, and the reflection amplitude of PP waves is stronger than that of PS waves. Therefore, the amplitude difference of PP and PS waves provides valuable information and feasibility for fluid identification. We verify that the maximum peak amplitude attribute of PS waves may better characterize the distribution of porous dolomite reservoirs than that of PP waves, and using the attribute of maximum peak amplitude difference between PP and PS waves, we may distinguish gas-bearing and water-saturated layers. Comparing with actual drilling results, we conclude that our PP/PS interpretation technique is feasible for fluid identification in deep carbonate reservoirs.

Formation water is the power and carrier of oil and gas generation, migration and accumulation, which has important significance for oil and gas migration, accumulation and preservation. The groundwater characteristics and genetic mechanism of the Permian Fengcheng Formation in Mahu sag are mainly studied through the chemical composition, isotope test, inclusion temperature measurement and salinity analysis. The study shows that the formation water of Fengcheng Formation in Mahu sag has special hydrochemical characteristics of high salinity, high HCO₃⁻ concentration, low Ca²⁺, Mg²⁺ concentration and high pH value NaHCO₃ water type. The characteristics of inclusion temperature measurement, carbon and oxygen isotope, strontium isotope and boron isotope show that the Fengcheng Formation fluid has deep hydrothermal source, and volcanism contributes significantly to its CO₂ source. The source of deep hydrothermal solution, volcanism and a large amount of hydrocarbon generation and expulsion from source rocks may be the main reasons for high HCO₃⁻ content in formation water. The Fengcheng Formation in Mahu sag is an alkalized lake deposit. The highest alkalization degree of water in the study area may be distributed in the southwest slope of Mahu lake. From the evolution stage of the alkali lake, the first member of the wind is the initial stage of the formation of the alkali lake, corresponding to the late stage of the high water level of the lake, and the water salinity is low; the second member of the wind is the strong alkali forming stage, corresponding to the Lake recession period, the water surface drops, the water body is strongly concentrated, and the salinity is high; the third member of the wind is the fading period of the alkali lake, corresponding to the early stage of the lake advance, the water salinity gradually decreases. On the whole, the groundwater mineralization degree of Fengcheng Formation in Mahu depression is high, and the formation sealing property is relatively good. The side near the edge of the basin is affected by the infiltration of atmospheric water, which results in the decrease of mineralization degree and the appearance of Na₂SO₄ type water. The water head decreases gradually from Mahu sag to northwest margin, and the gradient of water head near Wuerhe in the north section is relatively strong, which is not conducive to hydrocarbon accumulation and preservation. Baikouquan and Karamay areas are low potential areas, and the water head gradient becomes gentle, which is conducive to oil and gas accumulation.

Siliceous dolomite is developed at the top of the fourth member of Dengying Formation in Penglai gas field, Sichuan Province, and can be used as a good cap rock to seal oil and gas. In this study, based on post-stack seismic data mining and optimization, sensitive logging curve analysis is found to be the optimal prediction method, subdividing small layers to finely characterize thin-layer siliceous dolomite. It is found that the navigation-pyramid two-stage classification seismic data and the waveform indication simulation method based on RXO parameters have a good recognition effect on the characterization of thin-layer siliceous dolomite. Studies have shown that faults can provide ascending channels for deep silicon-rich hydrothermal fluids, resulting in the development of siliceous dolomite mostly near the fault zone. The siliceous dolomite in the upper sub-member of the fourth member of Dengying Formation in Penglai area mainly develops two sets of siliceous dolomite in the vertical direction, and the lower siliceous rock develops on a larger scale. In the lateral direction, it is mainly distributed in the JT1-PS7-P8-PS9 well area in the north, in a continuous sheet shape. In the south, the development scale of siliceous dolomite is small and distributed in the shape of a strip.