Factors influencing physical property evolution in sandstone mechanical compaction:the evidence from diagenetic simulation experiments

In order to analyze the factors influencing sandstone mechanical compaction and its physical property evolution during compaction processes, simulation exper- iments on sandstone mechanical compaction were carried out with a self-designed diagenetic simulation system. The experimental materials were modem sediments from dif- ferent sources, and the experiments were conducted under high temperature and high pressure. Results of the exper- iments show a binary function relation between primary porosity and mean size as well as sorting. With increasing overburden pressure during mechanical compaction, the evolution of porosity and permeability can be divided into rapid compaction at an early stage and slow compaction at a late stage, and the dividing pressure value of the two stages is about 12 MPa and the corresponding depth is about 600 m. In the slow compaction stage, there is a good exponential relationship between porosity and overburden pressure, while a good power function relationship exists between permeability and overburden pressure. There is also a good exponential relationship between porosity and permeability. The influence of particle size on sandstone mechanical compaction is mainly reflected in the slowcompaction stage, and the influence of sorting is mainly reflected in the rapid compaction stage. Abnormally high pressure effectively inhibits sandstone mechanical com- paction, and its control on sandstone mechanical com- paction is stronger than that of particle size and sorting. The influence of burial time on sandstone mechanical compaction is mainly in the slow compaction stage, and the porosity reduction caused by compaction is mainly con- trolled by average particle size.

CHARACTERISTICS AND DEVELOPMENT MODEL OF DUNE ROCKS ON SOUTH CHINA COASTS

Dune riocks are aeolian sands cemented ty calcium carbonate under subaerial conditions. They have been found in many of the coastal belts of Fujian, Guangdong and Hainan Provinces in South China. The grain composition of the dune rocks is mainly quartz sands and shell fragments. The quartz sands are medium and fine sized, relatively well sorted and positively skewed. Their surface texture formed in aeolian environments is characterized ty dishshaped depressions, meniscus depressions and V-shaped depressions with rounded edges. The most common bedding type of the rocks is larg (thickness>1.5m), steeply dipping (32--40°) with cross strata tolaner and convex upward). Mg and Sr contents are very low in the rock chemical composition which is classified into low Mg and low Sr category. The typical species of microfossils in the dune rocks are mainly freshwater ones and lack of typical saltwaer or semi-saltwater ones with incomplete assemblage of marine species. The cement minerals in the rocks are mainly low-Mg calcite and the common cement fabrics are meniscus cement and gravitational cement in response to impermanent water in vadose zones. Therefore, the dune rocks may be apparently distinguished from the beach rocks.

Characteristics and origin of high-quality lacustrine carbonate reservoirs in the gentle slope area of the Qikou Sag, Bohai Bay Basin

Lacustrine carbonate reservoirs in the gentle slope(ramp)area of Qikou Sag are highly heterogeneous.Some researches about characteristics and distribution of these high quality reservoirs are less.In this study,an integrated investigation was conducted on the high-quality lacustrine carbonate reservoirs in the ramp area of Qikou Sag based on data of thin section examination,lithological log response,testing results,and mercury-injection capillary pressure measurement.In the study area,grainstone reservoirs in the lower 1st member of the Shahejie Formation are high-quality reservoirs characterized by high-medium porosity,moderate permeability,low displacement pressure,and relative large pore throat.Lithologically,these reservoirs are mainly composed of bioclastic and oolitic sparite,and reservoir storage space consists of secondary intergranular dissolved pores and fractures.Hydrodynamic conditions generally control distribution of these reservoirs,as limestone grains filled intergranularly with sparry calcite cement are usually formed under strong hydrodynamic conditions.Sparry calcite cemented limestone is subject to late dissolution with abundant soluble substances,which is one of the direct causes for the well developed secondary dissolved pores.Oolitic bank and bioclastic bank are under high-energy hydrodynamic conditions with the best reservoir petrophysical properties.Rims of beach bars are usually in an intermittently-turbulent highly-hydrodynamic environment with medium reservoir petrophysical properties.Lake bays and supratidal zones are under low-energy tranquil hydrodynamic conditions,with poor reservoir petrophysical properties.During late diagenesis,large quantities of organic acid and slightly acidic water were released from surrounding shale and migrated along the top unconformity of the 3rd member of the Shahejie Formation,resulting in development of numerous secondary dissolved pores in carbonate reservoirs,which is the main cause of high-quality reservoir formation in the sag.Additionally,early charge of hydrocarbons restrained occurrence of authigenic minerals and metasomatism of calcite,and thus promoted excellent preservation of dissolved pores.