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Investigations directed toward petroleum exploration problems have shown that diagenetic parameters can be useful in mapping subtle structural features that are not otherwise apparent. The investigations are based on the premise that structural movement alters the primary character of a sediment and that the degree and extent of alteration are a function of: (1) type of movement, (2) local intensity of stress, (3) age of movement with respect to reference horizons, and (4) the number of movements. Emphasis for exploration purposes is placed on (a) analysis of the alteration within the objective or reservoir horizon, and (b) interpretation of the structural history of the objective horizon in terms of stratigraphy, textural distribution patterns, and alteration in the over ying strata.
At numerous locations in the northern Rocky Mountains, anomalous textural distribution patterns in strata of different ages are superimposed on each other. These patterns reflect the recurring influence of structural movement in the same place at different periods in geologic time. Having defined these trends in broad terms of sedimentation, the local structural anomalies can be defined more precisely by analyzing the diagenetic changes; i.e., changes in cementation, grain and crystal alteration, solution characteristics, and fracture filling. These parameters show by the stratigraphic position of the alteration the influence of compaction, downwarp, upwarp, flexing, folding, incipient faulting, and truncation.
Calcite to dolomite conversions, as mineral cement in detrital rocks or as host material, offer the most apparent type of change as an indicator of structural deformation. Examples in the Williston basin are shown from Nesson and Cedar Creek anticlines, and also from the projection of the down-warped flexure related to the Van Norman fault. Other local areas along the northern limb of the basin are presented for appraisal.
Recrystallized and injected forms of anhydrite and halite are used as supplemental criteria because of their instability under stress. As deterrents to permeability, they receive special consideration when encountered in the objective horizon. Examples are cited from the Nesson-Frobisher and Midale facies in the northern Williston basin and from the Minnelusa formation of Wyoming.
Authigenic chert and quartz mosaics in the host rock and in fractures are used to help define faulted zones and unconformable contacts related to uplift and erosion. Examples are cited from the post-Mississippian-pre-Triassic surface of central Montana.
Results of these and other investigations show that alteration criteria can help to localize exploration prospects, and that some of the better known "stratigraphic traps" in the northern Rocky Mountains have had a recurring history of structural advantage over the surrounding area throughout much of geologic time.
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