The Role of Meteoric Water in Diagenesis of Shallow Sandstones: Stable Isotope Studies of the Milk River Aquifer and Gas Pool, Southeastern Alberta
Fred J. Longstaffe, 1984. "The Role of Meteoric Water in Diagenesis of Shallow Sandstones: Stable Isotope Studies of the Milk River Aquifer and Gas Pool, Southeastern Alberta", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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Oxygen- and carbon-isotope compositions have been determined for clay and carbonate minerals from the Upper Cretaceous clastic rocks of the Milk River and Lea Park Formations. These units contain the Milk River aquifer and the southeastern Alberta Milk River Gas Pool, respectively.
The stable isotope data provide important information concerning the diagenesis and paleohydrology of the study area. Authigenic minerals from sandstones in the Milk River aquifer are characterized by low δ18O and δ513C values: clay minerals (<2 μm), dominated by authigenic kaolinite, δ18O = +11.3 to + 14.2 (SMOW); authigenic calcite, δ518O = +15.3 to +18.5 (SMOW), δ13C = −9.9 to −2.6 (PDB). The authigenic minerals with the lowest δ518 O values occur within a zone of local recharge in the aquifer. Here the authigenic clay minerals and calcite closely approach isotopic equilibrium with existing meteoric water at low temperatures (<+15°C). Low δ13C values forthe calcite indicate incorporation of organically derived CO2, probably from decaying plant material in the overlying soil and till.
Close agreement between actual formation temperatures and those calculated from isotopic data disappears in downdip portions of the aquifer, mostly because of the drastic enrichment in 18O of the formation water in this direction (−20 to −6, SMOW; Schwartz et al, 1981). Authigenic minerals from these locations have retained isotopic signatures characteristic of 18O-poor meteoric water no longer present in the system. This water was displaced by 18O-rich formation fluids that are themselves now being flushed from the aquifer by modern-day ground water.
Authigenic minerals from sandstones in the southeastern Alberta Milk River Gas Pool are more 18O-rich than those from the aquifer: clay minerals, dominated by illite, +14.7 to +15.8, SMOW; calcite, +19.3, SMOW. Such compositions are compatible with mineral crystallization at low temperatures (+15 to +20° C) from formation fluids similar in δ518O to other Cretaceous oil and gas pools that occur in Alberta. The low-13C nature of authigenic calcite and some dolomite (−7.6 to −3.0, PDB) from the Milk River Gas Pool may be related to the production of biogenic methane in this reservoir.
Of all clay minerals analyzed, the illite-dominated mixtures from argillaceous rocks of the Milk River aquifer and the Milk River Gas Pool have the highest δ18O values (+16.0 to +19.0, SMOW). Such compositions reflect a detrital origin rather than diagenetic processes. Isotopic exchange between these clay minerals and formation water is insignificant.
Most dolomite from the sandstones and the argillaceous rocks is not in equilibrium with the authigenic calcite. The dolomite is much richer in 18O (+24.4 to +28.3, SMOW) and 13C (−2.7 to +1.0, PDB); such values are typical of platform carbonate rocks. No evidence for extensive isotopic exchange between formation water and the dolomite can be demonstrated.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.