The Relationship Between Sandstone Diagenesis And Overpressure Fuild Of The Shahejie Formation In Dongying Depression

The overpressure compartment plays an important role in hydrocarbon generation, migration and accumulation in petroliferous basins, the overpressure system is well development within Shahejie Formation in Dongying Depression, whereas the mechanism of overpressure development, the process of carbonate mineralization near the top seals of clastic sedments and the effect of overpressure development to pore evolution is all insufficient understanding. By applying multiple means such as thin section identification, cathodoluminescene, electron microscope, X-ray, pore and permeability analysis, fulid inclusion test, C/O isotope detection and Basin modeling, we studied the development and the evolution of overpressure system, the relationship between overpressure fulid invasion and carbonate mineralization as well as porosity development. The main ideas are listed as follows.1. The overpressure compartment identified in the3rd-4th member of Shahejie Formation ranging from2000-4400m, and the depth of top seal increases from2000m to3000m coordinating with the burial of source rocks. The degree of sanstone overpressure increases with the content of oil bearing, indicating that the reservoir overpressure is origin from hydrocarbon generation which transmited from overpressured mudstones. The pressure calculated from fuild inclusions and basin modeling show that the early overpressure appears to be caused by undercompaction, there is only a limited contribution for hydrocarbon generation.2. The diagenetic sequence was determined as:(muddy clay or micrite carbonate) matrix filling→compaction→feldspar and lithic dissolution→kaolinite and quartz overgrowth→iron calcite cementation→iron calcite dissolution→ankerite cementation. Thin section particle size analysis show that the sorting coefficient of sanstones in the3rd&4th member of Shahejie Formation range from1.62to2.96with an average of2.00, whilst the original intergranular volume range from28.64%to35.05%with an average of32.63%according to the Beard equation Φ=20.91+22.90/Sd. Then we established the quantitative evaluation model of diagenesis: φ=F+axA+β×B, where F=31.894, α=-12.878, β=-32.029, with high goodness of fit R2=0.9393. Based on the quantitative evaluation equation, muddy clay or micrite carbonate matrix filling about-8.44%to0%of porosity, compaction and dissolution overall increasing about-5.22%to4.90%of intergranular volume, carbonate cements filling about-24.28%to-3.21%of porosity. The sanstone porosity can reduce as high as10%if intense carbonate cementation occurs, thus the carbonate cementations have the ability on controlling porosity development. 3. The burial depths of overpressure top boundary ranges from2200～2800m in Dongying Depression. Interacted with overpressure fluids that migrated beneath, the top of overpressure compartment is generally characterized by well development carbonate mineralization, commomly with carbonate content between15～40%. The statistical analysis on in-situ electron-microprobe compositional data indicated that the carbonate minerals can be subdivide into three groups, i.e. penecontemporaneous dolomite, iron calcite and ankerite, integrated with X-ray diffraction, Cathodoluminescence as well as other means, their diagenetic sequence was determined as penecontemporaneous dolomite→calcite→ankerite. On the basis of fluid inclusion test, the precipitation of carbonate mineral in the top seal and adajacent sandstones was accompanied with overpressure fluids, with a minimum paleopressure coefficient ranging between1.29～1.62and a precipitation temperature obviously higher than the background temperature, which suggests a significant influence by overpressured thermal fluid invasion. Thus, based on previous study results, we proposed that the precipitation of calcite and ankerite might be related to activities of overpressured fluids at the terminal Dongying stage and since the deposition of Minhuazhen stage, respectively. Furthermore, this hypothesis was tested by oxygen isotope data that fluctuate from-16.86%o to-12.29%oPDB for calcite and from-12.20%o to-10.20%oPDB for ankerite. Further investigations suggest that the calcite should precipitated at90～120℃for the δ18OSMOW value of it’s homochronous overpressured fluid is around0.00‰, and the ankerite should precipitated at110～135℃for the δ18OSMOW value of it’s homochronous overpressured fluid is around0.25‰.δ13C values of carbonate cements formed at the late diagenesis show a positive shift, ranging from-0.9%o to+3.58%o, which indicates an origin mainly from the dissolution-reprecipitation process of lacustrine carbonate, or suggests the influence of intramolecular isotopic fractionnation for carbon isotope in carboxylic acids, though the latter seems to play a relatively minor role.4. Early overpressure can resist the compaction process and extend significant positive effects on the reservation of sandstone porosity. The inhibition degree suggests to be determined by the intensity of overpressure, the greater magnitude of the overpressure fluid developed, the lower compaction of the sandstone performed. The iron calcite that seems to be cemented during early overpressure fluids releaseing performs dual-influences on porosity evolution, it can significant reduce porosity in one hand and also can reserve a certaion mount of intergranular space which maybe released by dissolution happening latter. Late overpressure fluids can help resisting compaction and bring organic acids for late dissolution, but the forming of large amount of ankerite can extremely damage porosity property. According to the statistical results of the measured data for pure lithology reservoirs, late oil-bearing overpressured fluid invasion can increase the porosity of sandstone reservoirs overall.