Search

The distribution, origin, and evolution of overpressure in the petroliferous basin are important problems that need to be addressed for oil and gas exploration. The distribution and origin of overpressure in the Shahejie Formation in the northern Dongpu Depression are analyzed based on geological studies, logs, and pressure data. The contribution ratios of different overpressure origins are quantified, and the evolutionary stages of overpressures of different origins are further divided. The results show that the formation pressure coefficients of Shahejie Formation in the Dongpu Depression are mainly within the range of 0.9–1.5. The overpressures are mainly distributed in the Sha-3 and Sha-4 Members of the Haitongji sag, the Central uplift belt and the Qianliyuan sag. From the sag to its surrounding area, the formation pressure coefficient gradually decreases. The high deposition rate and strong hydrocarbon generation are the main causes of overpressure formation in the Shahejie Formation in the Dongpu Depression. Based on the stress–strain characteristics of different origin overpressures and the log response parameters, two models, acoustic travel time-effective vertical stress and electrical resistivity-effective vertical stress, are established to identify and quantify the different origin overpressures. The calculation results for the area from the Haitongji sag and Qianliyuan sag to the Central uplift belt show that the main cause of overpressure gradually changes from both disequilibrium compaction and hydrocarbon generation to disequilibrium compaction as the main factor, with the contribution of disequilibrium compaction to overpressure in the Central uplift belt at about 87%. The Sha-3 Members of the Shahejie Formation in the Haitongji sag and the Qianliyuan sag are more strongly affected by hydrocarbon generation, which accounts for 42% and 47.5% of overpressure origin, respectively. There are five stages in the evolution of overpressure in the Shahejie Formation in the northern Dongpu Depression: normal compaction (before 35 Ma), mixed pressurization (35-27 Ma), uplift and pressure release (27-17 Ma), disequilibrium compaction (17-11 Ma), and secondary mixed pressurization (12 Ma-present). • Overpressure mainly distributes in the Sha-3 and Sha-4 Members of the Dongpu Depression. • Disequilibrium compaction and hydrocarbon generation are main overpressure origins. • Two models to identify and quantify different origins' overpressure were came up. • Overpressure origin changes in different tectonic units and evolution stages. [ABSTRACT FROM AUTHOR]

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe2+ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe2+ and Mg2+ for Ch3 growth in sandstones. Chlorite is considered to have "composite" impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir. • Three different types of chlorite have been found in the Xujiahe tight sandstones. • It has been clear that the formation time of chlorite and the paragenetic sequence of authigenic minerals. • The origin and formation mechanism of authigenic chlorite in three types have been investigated. • We considerd that authigenic chlorite has a "composite" impact on deep tight sandstone: preserve primary porosity but reduce the permeability. [ABSTRACT FROM AUTHOR]