A great amount of quartz was found filling vugs of the dolomite strata of Upper Cambrian-Lower Ordovician in Keping(柯坪) outcrop area of Tarim basin,China,which is widely distributed with obvious impact on dolomite reservoirs.The quartz's genesis and environment were studied based on careful field observation and indoor comprehensive analysis.The abnormal high temperature of the primary saline aqueous inclusions of quartz minerals might reach as high as 578 ℃,showing the characteristics of basin abnormal heat events.The quartz-rare earth element(REE) pattern is similar to those of the granite,diabase,and basalt of Permian in Tabei(塔北) area.According to the regional geological setting and quartz's distribution as well as its occurrence,the genesis of quartz is attributed to Permian magmatic activity-related hydrothermal liquid instead of the primary marine sedimentary genesis or regional metamorphic genesis.The δ 13 C values of CO 2 in the quartz inclusions range from-1.4‰ to-4.6‰,which might derive from the mantle,while δ 18 O of SiO 2 indicates a hot fluid genesis of meteoric precipitation,which consists of the meteoric precipitation diagenetic setting of C and O isotopes in the dolomite host rocks and the low-medium salinity freshwater condition in the inclusions.REEs must have experienced clear differentiation,since the values of LREE/HREE are all over 1,clearly indicating the characteristic of enrichment of LREE and decrement of HREE.Ce has no obvious anomaly and Eu has clear negative anomalies,which suggests a reducing environment when the quartz crystallized.After the comprehensive analysis,we believe that quartz is a product of siliceous hydrothermal liquids in combination with meteoric precipitation under a reducing environment.Additionally,the precipitation hydrothermal fluid mixture might have also mixed thoroughly with the formation water and exchanged material with the host dolomites.The significance of siliceous hydrothermal fluids for dolomite reservoirs as well as oil-gas exploration
The Carboniferous Karashayi(卡拉沙依) Formation of Tarim basin formed in the epicontinental sea shelf environment where the evolution of basins is pediocratic.As an important reservoir-forming combination,the Carboniferous Karashayi Formation consist of clastic rocks,carbonate rocks and gypsum rocks,which show clear sedimentary cyclicity.According to paleontology research and stratigraphic correlation,Karashayi Formation corresponds to Visean and Serpukhovian in international stratigraphic chart with a time limit about ±27 Ma.The sequence stratigraphic study for the Karashayi Formation is rough because of the difficulty to identify the unconformity surfaces.The current study mostly divides it into three or four 3rd sequences.However,this partition cannot meet the geological crossing correlation.At the theory aspect of sequence stratigraphy,the genesis of the 3rd sequence is also a disputed problem.Cyclostratigraphy study of the Natural Gamma-Ray Log,Spontaneous Potential Log and other logging curves has been taken out by spectrum analysis and wavelet analysis,etc..For Well Shun6 and Well Zhong1 in Tazhong(塔中) and neighbouring area,combined core observation and base level analysis,the Karashayi Formation was divided into 9 or 11 sequence stratigraphic units.Continuous constraints of each sequence is about ±2.40 Ma,corresponding to astronomical cycle formed by three parameters of Earth orbit.Accompanied by the historical division of 3rd sequences,the formation mechanism of 3rd sequences was possibly forced by the 2.4 Ma astronomical eccentricity cycles.
The reef-shoal depositional system of the Ordovician carbonate platform margin is well exposed in the Yijianfang(一间房) outcrop of the Bachu(巴楚) uplift region,which offers an advantageous condition to study their paleoecology.Using a detailed field geologic survey and illustrated profiles of typical depositional systems,three types of genetic facies associations can be recognized in the reef-shoal depositional system:an organic reef,an organic shoal,and an upper slope.The organic reef is composed of three types of genetic facies(a reef base,a reef core,and fore-reef breccias);the organic shoal is formed from five types of genetic facies(tide channels,fore-reef inner shoals,fore-reef outer shoals,back-reef inner shoals,and back-reef outer shoals).The studies of the paleontological assemblage in each genetic facies of the depositional system indicate that the fauna preserved in each genetic facies are varied.The calathium,archaeoscyphia,bryozoan,and calcareous alga are well preserved in the organic reefs.The organisms preserved in the organic shoals are generally fragmented,while well-preserved girvanella and nuia siberica with a content of about 15% in the back-reef outer shoals are the most characteristic and different from others.The sinoceras,trilobites,and gastropods are well preserved in the upper slope deposits.The studies will demonstrate that the reef-shoal complexes developed above the base of the fair-weather wave base and that the original hydrodynamic conditions for the reef core forming is the stronger and become more and more low-energy from the inner part to outer part of the organic shoals.
The oolitic shoals of the Triassic carbonate platform margin in the Yudongzi(鱼洞子) outcrop of Erlangmiao(二郎庙) area in the northwestern Sichuan(四川) basin present a scarce opportunity to quantitatively describe their diagenesis and its effects on the acoustic velocity.Using a detailed field geologic survey,profiles illustration of typical depositional system,and systematic testing,five types of diagenesis have been identified in the oolitic shoals:micritization,cementation,compaction and pressolution,dissolution,and dolomitisation.The cementation is composed of four subtypes(micrite cements,fibrous calcite cements,granular calcite cements,and blocky calcite cements).The dissolution is formed from three subtypes(freshwater selective dissolution,burial non-selective dissolution,and burial selective dissolution).The dolomitisation is composed of three subtypes(fine-crystalline dolomites,microcrystalline dolomites,and medium-crystalline dolomites).In order to quantitatively describe the diagenetic fabric of oolitic shoals,the micritic grain content,calcite cement content,mean pore diameter,pore types,dolomite content,and dolomite types have been evaluated.Based on these data,the relationship between the acoustic velocity and diagenesis of oolitic shoals has been established.The results show that the diagenetic fabric is linearly related with the acoustic velocity,and the general trend observed is as expected a decrease of velocity as the micritic grain content,mean pore diameter and dolomite content increase,or the sparite cement content decreases.This study will demonstrate that the transformation of diagenetic facies will probably make the petrophysical properties of the oolitic shoals regularly changed.The reflection configuration of diagenetic facies in the oolitic shoals can be shown in the synthetic seismic model simulated according to the P-wave impedance and S-wave impedance.
The Cenozoic reef-banks reservoir in Qiongdongnan(琼东南) basin is one of the most profitable targets worthy of prospecting in the deep water area of the northwestern South China Sea.In this study,the characteristics of organic reef-like reflectors in southern uplift area of Qiongdongnan basin are analyzed based on the latest 2D and 3D seismic data.It is found that reef-like reflectors in Qiongdongnan basin show high-amplitude moundy continuous reflection at the top and the bottom,chaotic reflection inside the reef-like reflector and there is obviously speed difference between reef-like reflector body and surrounding rock.Combining the geological setting of reef and comparative analysis,the reef-like reflectors are considered to be the reef-banks.Furthermore,the results show that there are three kinds of reef structures in the southern uplift area of Qiongdongnan basin:the fault controlling structure,the fault flexure structure and the carbonate ramp structure.Each structure has its own grow-units that can reveal the growth process of the reef in order.The distribution of reef-like reflectors in space shows an arc belt pattern.According to this,four reef arc belts are identified,and their spatial trends are thought to have a good relationship with the buried fault.The internal structure constitution and the growth state of reef-like reflectors are mainly controlled by the geomorphology and monsoon-ocean current.The result has an instructive significance to oil-gas exploration of organic reef not only in Qiongdongnan basin,but also even in deep water areas of the South China Sea.
