| Popis: |
SPE Members Abstract Geochemical and geophysical well log data can be used to discriminate between clay-supported and framework- (or grain-) supported domains on the basis of observed clay and porosity relationships. The existence of these domains in unconsolidated materials has been inferred from experimental, bimodal mixtures of quartz and kaolinite but has not been demonstrated in consolidated rocks. The grain supported domain is characterized by decreasing porosity and increasing compressional velocity with increasing clay content. The transition to the clay-supported domain is marked by a reversal or flattening of these trends in porosity and velocity. In the clay supported regime, increases in clay content correspond to increases in porosity and decreases in compressional velocity. The existence of the two domains can be identified by modified v-shaped patterns in clay porosity crossplots or by inverted v-shaped patterns in clay velocity crossplots. Once the patterns have established for a given section of a well, they can be statistically analyzed, and then the clay volumes can be used to create synthetic logs of porosity and compressional velocity. Outside the range where the statistical fit is performed, deviations between the synthetic logs and the measured logs may be interpreted as geological discontinuities. These discontinuities arc most likely due to the diagenetic history of the rocks, and in most cases probably represent time horizons. Introduction Recently published work has shown that relationships between clay content, porosity, and compressional velocity can be used to discriminate between framework-supported and matrix supported domains in laboratory mixtures of quartz sand and kaolinite. This paper uses geochemical and geophysical log data to examine the existence of such relationships in both unconsolidated and consolidated sedimentary rocks. Examples from several Gulf of Mexico wells show patterns similar to those observed in the experimental data. Evidence of sedimentary compaction can be seen in both the relationships between clay and porosity, and clay and compressional velocity. An example from consolidated sedimentary rocks reveals remarkably clear trends in the porosity-clay-velocity relationships. As in the unconsolidated rocks, these trends suggest the existence of two domains: one which is the diagenetic descendant of a grain-supported sand, and the other which has diagenetically evolved from matrix-supported sediments. In such consolidated rocks, it is necessary to consider the impact of chemical diagenesis in addition to compaction. In the example presented here, it is possible to use geochemical logs to identify and to a limited extent interpret the role of carbonate cementation in the diagenesis of the rocks. The experimental work of Marion et al. also graphically presents the impact of increasing pressure on the experimental samples. If the pressure or compaction history is reflected in the porosity-clay-velocity relationships in the rocks, then it is reasonable to use those relationships to look for geologic discontinuities or reservoir phenomena such as overpressuring. Examples of apparent geologic discontinuities are presented from two wells. P. 667^ |
