Formation of fine crystalline dolomites in lacustrine carbonates of the Eocene Sikou Depression, Bohai Bay Basin, East China

The genesis of the fine crystalline dolomites that exhibit good to excellent reservoir properties in the upper fourth member of the Eocene Shahejie Formation（Es4s）around the Sikou Sag, Bohai Bay Basin, is uncertain. This paper investigates the formation mechanisms of this fine crystalline dolomite using XRD, SEM, thin section analysis and geochemical data. The stratigraphy of the Sikou lacustrine carbonate is dominated by the repetition of metre-scale, high-frequency deposition cycles, and the amount of dolomite within a cycle increases upward from the cycle bottom. These dolomite crystals are 2–30 μm in length, subhedral to anhedral in shape and typically replace both grains and matrix. They also occur as rim cement and have thin lamellae within ooid cortices. Textural relations indicate that the dolomite predates equant sparry calcite cement and coarse calcite cement. The Sr concentrations of dolomites range from 900 to 1200 ppm. Dolomite δ18O values（-11.3 to-8.2 ‰ PDB） are depleted relative to calcite mudstone（-8.3 to-5.4 ‰ PDB） that precipitated from lake water, while δ13C values（0.06–1.74 ‰ PDB） are within the normal range of calcite mudstone values（-2.13 to 1.99 ‰ PDB）. High87Sr/86Sr values（0.710210–0.710844） indicate that amounts of Ca2＋ and Mg2＋have been derived from the chemical weathering of Palaeozoic carbonate bedrocks. The high strontium concentration indicates that hypersaline conditions were maintainedduring the formation of the dolomites and that the dolomites were formed by the replacement of precursor calcite or by direct precipitation.

Sedimentary filling characteristics and controlling factors of lacustrine microbial carbonates sequence in the Santos Basin,Brazil

Based on comprehensive analysis of seismic,logging,core,thin section data,and stable isotopic compositions of carbon and oxygen,the sedimentary filling characteristics of the Lower Cretaceous Barra Velha Formation sequence in H oil field,Santos Basin,are studied,and the high-frequency sequence stratigraphic framework is established,and the spatial distribution of reef-shoal bodies are predicted and the controlling factors are discussed.During the depositional period of the Barra Velha Formation,the study area is a slope-isolated platform-slope sedimentary pattern from southwest to northeast and the change of climate background from rift to depression periods has resulted in the variation of sedimentary characteristics from the lower third-order sequence SQ1(BVE 300 Member)of low-energy deep water to the upper third-order sequence SQ2(BVE 200 and 100 members)of high-energy shallow water in the Barra Velha Formation.The activities of extensional faults and strike-slip faults in rift period and the sedimentary differentiation from platform margin to intra-platform in depression period made the sedimentary paleogeomorphology in these two periods show features of“three ridges and two depressions”.The reef-shoal bodies mainly developed in the SQ2-LHST period,with vertical development positions restricted by the periodic oscillation of the lake level,and developed on the top of each high-frequency sequence stratigraphic unit in SQ2-LHST in the platform.The strike-slip fault activity controlled the distribution of the reef-shoal bodies on the plane by changing the sedimentary paleogeomorphology.The positive flower-shaped strike-slip faults made the formation of local highlands at the margins of and inside the shallow water platforms and which became high-energy sedimentary zones,creating conditions for the development of reef-shoal bodies.

Mixed carbonate rocks lithofacies features and reservoirs controlling mechanisms in a saline lacustrine basin in Yingxi area,Qaidam Basin,NW China

Taking the mixed pre-salt carbonate rocks in the upper member of Eocene Xiaganchaigou Formation(E_(3)^(2))of Yingxi area in the Qaidam Basin as an example,the lithofacies and controlling mechanisms of reservoir formation are analyzed based on a large dataset of cores,thin sections and geochemical analysis.The reservoirs in E_(3)^(2)pre-salt layers have five types of lithofacies,of them,mixed granular calcareous dolostone,massive calcareous dolostone,plaque calcareous dolostone,and laminated dolomtic limestone are of sedimentary origin,and breccia calcareous dolostone is of tectonic origin.The four types of sedimentary lithofacies are divided into two types of saline sedimentary sequence lithofacies combinations,low-energy type in the sag area and low to high-energy type in the slope and paleo-uplift zone in the depression.Affected by high-frequency supply of continental clastic material,the two types of salty sedimentary sequences are mostly incomplete subtypes of lithofacies.Lithofacies have strong impacts on pre-salt reservoirs in E_(3)^(2):(1)Lithofacies type and sedimentary sequence controlled the formation and distribution of dolomite intercrystalline pores and dissolved pores during the pene-sedimentary period.(2)The structure of laminated dolomitic limestone controlled the formation of large-scale laminated fractures and high permeability channels during the diagenetic period.(3)Granular,massive,plaque calcareous dolostones have low mud content and strong brittleness,in the late tectonic reactivation period,the distribution of the three types of lithofacies,together with their distance from the top large slip faults and secondary faults,controlled the formation and distribution of high-efficiency fracture-cave brecciaed calcareous dolostone reservoirs.The above research led to the composite lithofacies-tectonic formation model of pre-salt reservoir in E_(3)^(2)of Yingxi area.The tempo-spatial distribution of tectonic breccia calcareous dolostone reservoirs,laminated dolomitic limestone shale oil reservoirs and granular,massive calcareous dolostone dissolved-intercrystalline pore tight reservoirs in various structural belts of the studied area have been figured out.These findings gave new insights into tight-shale oil accumulation theory in mixed carbonate successions from saline lacustrine basins,aiding in high efficient exploration and development of petroleum in the studied area.

Theoretical and technical progress in exploration practice of the deep-water large oil fields, Santos Basin, Brazil

The history and results of petroleum exploration in the Santos Basin, Brazil are reviewed. The regularity of hydrocarbon enrichment and the key exploration technologies are summarized and analyzed using the seismic, gravity, magnetic and drilling data. It is proposed that the Santos Basin had a structural pattern of two uplifts and three depressions and the Aram-Uirapuru uplift belt controlled the hydrocarbon accumulation. It is believed that the main hydrocarbon source kitchen in the rift period controlled the hydrocarbon-enriched zones, paleo-structures controlled the scale and quality of lacustrine carbonate reservoirs, and continuous thick salt rocks controlled the hydrocarbon formation and preservation. The process and mechanism of reservoirs being transformed by CO_(2)charging were revealed. Five key exploration technologies were developed,including the variable-velocity mapping for layer-controlled facies-controlled pre-salt structures, the prediction of lacustrine carbonate reservoirs, the prediction of intrusive/effusive rock distribution, the detection of hydrocarbons in lacustrine carbonates, and the logging identification of supercritical CO_(2)fluid. These theoretical recognitions and exploration technologies have contributed to the discovery of deep-water super-large reservoirs under CNODC projects in Brazil, and will guide the further exploration of deep-water large reservoirs in the Santos Basin and other similar regions.

Clumped isotope analysis of lacustrine endogenic carbonates and implications for paleo-temperature reconstruction:A case study from Dali Lake

Carbonate clumped isotope(Δ47)is a new and reliable geothermometer.Endogenic carbonates in lake sediments are good archives to reconstruct lake water temperature using clumped isotope thermometry.However,applications of carbonate clumped isotope thermometry to lacustrine carbonates are still scarce because the existed organic and/or sulfur contaminants in such samples interfere with clumped isotope analysis and cause notable temperature biases.Therefore,exploring an effective way to remove contamination is a prerequisite to widely applying carbonate clumped isotope thermometry to lake sediments.By pretreating fine-grained endogenic carbonates from Dali lake sediments with hydrogen peroxide of different concentrations for different lengths of time,we conducted a series of conditional experiments to seek the optimal pretreatment condition for clumped isotope analysis.Δ47,Δ48 offset and 49 parameter were obtained from clumped isotope measurements to assess the effect of contamination removal.Results showed that untreated samples("0%-0")had a very high 49 parameter.After the treatment with hydrogen peroxide,the 49 parameter was significantly reduced while theΔ48 offset evidently increased.TheΔ47,Δ48 offset and 49 parameter of samples pretreated under different conditions showed significant variability,even though the measuredΔ47 temperatures changed within a relatively narrow range(i.e.,from 17.0±1.3 to 22.8±1.6°C).Among various treated samples,the sample"3%-8"yielded a smallerΔ48 offset and 49 parameter,and lowerΔ47 temperature of 17.0±1.3°C.Therefore,"reacting with 3%hydrogen peroxide for 8 h"was suggested to be the optimal condition for removing contaminants from lacustrine endogenic carbonates prior to clumped isotope analysis.At the time when the pretreatment condition was too intense(i.e.,H2O2 concentration>3%and/or reaction time>8 h),secondary contaminants might have been generated in the closed reaction system,whose ultimately transformed state may interfere with masses 47,48.Using the optimal pretreatment procedure,we obtained reasonableΔ47 temperature changes during the last deglaciation and found a temperature decrease of about 6°C in Northern China during the Younger Dryas period.Our study demonstrates a great potential of applying carbonate clumped isotope thermometry to lacustrine carbonates for paleo-temperature and paleo-elevation reconstructions in the future.

Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai Member, Lower Jurassic, Beibei area, eastern Sichuan Basin

There are few reports about the penecontemporaneous karst of the lacustrine carbonate of Da'anzhai Member which developed on the top of Lower Jurassic Ziliujing Formation,eastern Sichuan Basin.Based on the analysis of the macroscopic and microscopic characteristics of Heping Reservoir section in Beibei area,It is discovered that the penecontemporaneous karst in the lacustrine carbonate,Da'anzhai Member,and its characters as follows:(1)The dissolved fractures and karrens are common,which have been filled with vadose silts,bioclasts,terrigenous quartzes,crystal and intergrown calcites;(2)The penecontemporaneous karstification mainly occurs in the micrites and shell limestones of the middle and upper part of the shallowing-upward sequence,and the desiccation cracks occur in some shallowing-upward sequences;(3)Three meteoric-water diagenetic lenses are identified in this section.By further study we also find the factures,stylolite and burial dissolution pores occur along the karst system(tubes,conduits and caves of the karst).And the asphalt not only fills the stylolites and fractures,but also is mixed with the vadose silt in the karst system,although the pore space formed by penecontemporaneous karstification is filled with vadose material and cements.Therefore it is concluded that the development and modification of the tight lacustrine carbonate reservoir of Da'anzhai Member,to some extent,are relevant with the penecontemporaneous karstification,and the penecontemporaneous karst reservoir may develop in the buried Da'anzhai carbonate of the basin.

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.