USE OF "ROCK-TYPING" TO CHARACTERIZE CARBONATE RESERVOIR HETEROGENEITY

The objective of the project was to apply techniques of "rock-typing" and quantitative formation evaluation to borehole measurements in order to identify reservoir and non-reservoir rock-types and their properties within the "C" zone of the Ordovician Red River carbonates in the northeast Montana and northwest North Dakota areas of the Williston Basin. Rock-typing discriminates rock units according to their pore-size distribution, Formation evaluation estimates porosities and pore fluid saturation. Rock-types were discriminated using cr6ssplots involving three rock-typing criteria: (1) linear relationship between bulk density and porosity, (2) linear relationship between acoustic interval transit-time and porosity, and (3) linear relationship between acoustic interval transit-time and bulk density. Each rock-type was quantitatively characterized by the slopes and intercepts established for different crossplots involving the above variables, as well as porosities and fluid saturations associated with the rock-types. Another family of linear relationships involving shear wave velocity, compressional wave velocity, and porosity were used to characterize the entire "e" zone carbonate section. Slopes and intercepts derived in the combined use of shear and compressional wave velocities are characteristic of carbonates, and corroborate results predicted from other studies. The "e" zone of the Red River carbonates is extremely heterogeneous. The heterogeneities were observed in hand specimen, and corroborated by wide variability's in porosity, permeability, grain density, the porosity-permeability crossplot, and the results from rock-typing and quantitative formation evaluation. Sixty-four different rock-types were identified in the fifty-two wells studied. Vertical distribution of rock-types shows a non-porous anhydrite rock-type, underlain, in most wells, by combinations of anhydritic dolomite and dolomite rock-types. These in turn are underlain by dolomitic limestone and limestone rock-types. In a few cases, dolomitic limestone rock-types directly underlie the anhydrite rock-type. The anhydritic dolomite and dolomite rock-types are either porous or non-porous, and in some wells interlayered with dolomitic limestone and limestone rock-types. Dolomitic limestone and limestone rock types are generally non-porous or have low porosities. The thickness of the non-porous dolomite rock-types varies from a few feet in some wells to over hundred feet in others. Except for the anhydrite rock-type that is present in all the wells, lateral distribution of rock-types is difficult to correlate, in part because of the wide separation between wells used in the study, and in part because rock-types change rapidly even in wells that are closely spaced. The occurrence of both porous and non-porous dolomite rock types, the random variation of their thickness, and the interlayering of these rock-types with limestone and dolomitic limestone rock-types are indicative of complex depositional environments for the "C" zone. Development of depositional models that would satisfy all of these conditions is beyond the scope of this study. All the existing production was confirmed through quantitative formation evaluation. Several potential productive zones were bypassed in both producing wells and those designated plugged-and-abandoned. Highly porous dolomites and anhydritic dolomites contribute most of the production, and constitute the best reservoir rock types. Dolomitic limestones and limestones make up the bulk of the untested and/or by-passed reserves. If all the bypassed zones proved correct, they represent approximately 150 million barrels of oil-in-place. Forty-six of the sixty-four rock-types identified developed oil reservoir characteristics in one or more of the fifty-two wells studied. Commonly, rock-types that were calculated to have pessimistic water saturations or lacked good porosity in one well, developed good porosities or produced oil in other wells. This observation indicates that the presence of oil was not an important factor in the discrimination of rock-types. Only twenty-six of the forty-six reservoir rock-types occur in perforated intervals. The rest were bypassed. In a few cases, all the oil produced came from a single rock-type, thus confirming the reservoir quality of the particular rock-type. In most other cases, more than one rock-type was contained in the perforated and producing interval, leading one to guess as to which rock-types were productive. The process of quantitative formation evaluation developed in this study proved consistent in identifying proven reserves. It could become the much needed approach for evaluating the Red River carbonates to determine fluid saturations and identify bypassed reserves. The results of this study can be applied in field development and in-fill drilling. Potential targets would be areas of porosity pinch outs and those areas where highly porous zones are down dip from non-porous and tight dolomites. Such areas are abundant. In order to model reservoirs for enhanced oil recovery (EOR) operations, a more localized (e.g. field scale) study, expanded to involve other rock-typing criteria, is necessary.

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