Depositional System of the Carboniferous Huanglong Formation,Eastern Sichuan Basin:Constraints from Sedimentology and Geochemistry

Using analyses of the lithology,sequences,paleoenvironment,and tectonic setting,the depositional system of the Carboniferous Huanglong Formation in the eastern Sichuan Basin was identified.The lithological characteristics of the Lower Member,Middle Member,and Upper Member were analyzed and classified.Before the use of carbon,oxygen,and strontium isotopes in the analysis,all of the geochemical data were tested for validity.On the basis of the Z values obtained from carbon and oxygen isotopes,the paleoenvironments of the three members were elucidated.Lower Member was dominantly an enclosed marine environment with intense evaporation and little freshwater input into the sea.Middle Member developed in a semi-enclosed to normal marine environment with many rivers.Upper Member was formed in a normal marine environment.The east Sichuan Basin was enclosed by paleouplifts before the deposition of the Huanglong Formation,forming a relatively enclosed depositional setting.Paleogullies developed in the Silurian strata that underlie the Carboniferous rocks;these paleogullies can be identified.On the basis of a comprehensive analysis,we propose that the Huanglong Formation developed in a platform system.Four microfacies were identified：supratidal flat,dolostone flat,grain shoal,and shelf microfacies.The high-permeability and high-porosity characteristics of the grain shoal microfacies are favorable for hydrocarbon accumulation,while the supratidal flat and shelf microfacies developed very few high-quality reservoirs.The paleogullies,in which increased amounts of grain shoal microfacies developed,controlled the distribution of high-quality reservoirs.

Geochemical characteristics and diagenetic fluids of dolomite reservoirs in the Huanglong Formation, Eastern Sichuan Basin, China

Based on a comprehensive study of texture,diagenetic behavior and evolution of dolomite in the Huanglong Formation,trace （e.g.,Fe,Mn and Sr） and rare earth element （REE） geochemistry,andisotopic characteristics （e.g.,C,O and Sr）,four types of diagenetic fluids are identified in the Huanglong Formation dolomite reservoirs of the Eastern Sichuan Basin,China：1）：marine-derived pore waters in the marine diagenetic environment,2） sabkha compaction brine conserved in the early shallowburied diagenetic environment,3） strongly-oxidizing low-temperature meteoric water in the seepagesubsurface flow diagenetic environment,and 4） strongly reducing deeply seated mixed hot brine in the middle and deep burial diagenetic environment.The fluids developed hereditarily from one environment to another,which resulted in its respective characteristics.Fluid characteristics play an important role in the development of dolomite reservoirs：1） dolomitization by marine-derived pore water in the quasisyngenetic stage did not form an effective reservoir; 2） early diagenetic burial dolomitization by the sabkha compaction brine formed the basis for reservoir development; 3） meteoric water karstification in the paleo-epidiagenetic stage expanded both the distribution and the size of the reservoirs,and improved the reservoir quality; 4） deep-burial dissolution and tectonic fracturing in the reburial diagenetic stage further improved reservoir porosity and permeability.

The Origin of Paleokarst in the Huanglong Formation of the Eastern Sichuan Basin:Evidence from δ^(13)C, δ^(18)O and ^(87)Sr/^(86)Sr

Karst rocks from the Huanglong Formation exposed at the margin of the Eastern Sichuan Basin can be divided into four types：slightly corroded, moderately corroded porous, intensely corroded brecciated and intensely corroded and replaced secondary calcic karstic rocks. The carbon, oxygen and strontium isotope compositions of the various karst rocks are analyzed systematically and compared to rocks without karst corrosion. The results indicate that（1） the Huanglong Formation in the eastern Sichuan Basin was a restricted bay supplied and controlled by freshwater in which mudmicrite and mud-dolomicrite exhibit low δ13C and δ18O values and high 87Sr/86 Sr ratios;（2） all types of karstic rocks in the paleokarst reservoirs of the Huanglong Formation in the research area are affected by atmospheric freshwater with the δ13C and δ18O values and 87Sr/86 Sr ratios in the original formation approaching those of atmospheric freshwater, which reflects ancient hydrological conditions, fluid properties, isotopic source and the fractionation effect;（3） the intensely corroded and replaced secondary limestone is affected by a variety of diagenetic fluids, often reflected by δ13C and δ18O values, while the 87Sr/86 Sr ratios exhibit the strong degree of the corrosion;（4） after comparing the 87Sr/86 Sr ratios of each type of karst rock, the diagenetic fluids are determined to be mainly atmospheric freshwater, and depending on the strength of corrosion, and the low 87Sr/86 Sr ratio fluids in the layer will participate in the karst process. The carbon, oxygen, and strontium isotopes of different karstic reservoirs can provide meaningful geochemical information for forecasting and evaluating the development and distribution rules of the Huanglong Formation at the margin of the eastern Sichuan Basin in time and space.