Digitized Outcrop Geomodeling of Ramp Shoals and its Reservoirs： as an Example of Lower Triassic Feixianguan Formation of Eastern Sichuan Basin

Numerous hydrocarbon accumulations are found in ramp crest shoals worldwide and therefore this depositional setting has a high potential of being the hydrocarbon reservoir.In this paper,we combined digital outcrop geology and traditional geological mapping to build an outcrop-based geocellular model of the ramp-crest shoal complex of the Lower Triassic Feixianguan Formation in the Eastern Sichuan Basin.The outcrop model serves as an analogue for the subsurface reservoir of the Feixianguan Formation and illustrates the complexity of the lithofacies types,stratigraphic architecture,and reservoir heterogeneities at a scale below conventional subsurface data resolution.The studied ramp-crest shoal complex consists of thirteen types of lithofacies that can be grouped into three facies-groups corresponding to subtidal intraclastic shoal,sub-to inter-tidal oolitic shoal,and tidal flat depositional environments respectively.The stratigraphic architecture of the shoal complex shows mostly a strong progradation of the high energy facies associated with an overall decrease of accommodation space associated with relative sea level still stand.Two reservoir facies associations have been recognized.The first one consists of supratidal dolomudstone and upper intertidal partially dolomitized oolitic packstone with anhydrite or nodules.These facies were deposited above the high energy oolitic grainstones and occurs as thin-bedded and laterally continuous layers,characterized by high porosity and low permeability.The second reservoir facies association is composed of intertidal crystalline dolomite and subtidal intraclastic bindstone that occurs stratigraphically below the oolitic grainstones.These deposits consist of massive laterally discontinuously beds,and are characterized by high porosity and high permeability.Both types of reservoir facies tend to be stacked vertically and migrated laterally with the progradation of the shoal complex.The construction of the outcrop-based 3D geological model provide a description and quantification of the facies distribution within a robust stratigraphic framework and the style and amount of reservoir heterogeneities associated with a ramp-crest shoal complex reservoir such as the one found in Lower Triassic Feixianguan Formation and Cambrian Longwangmiao Formation in Sichuan Basin or other ramp-crest reservoir worldwide.

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, and isotopic 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.