Browsing by Subject "Cementation (Petrology)"
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Item The genetic association between brittle deformation and quartz cementation: examples from burial compaction and cataclasis(2004) Makowitz, Astrid; McBride, Earle F.; Milliken, Kitty L.Brittle deformation of quartz grains accompanied by quartz cementation is a porosity-reducing mechanism in sandstones. Brittle deformation has historically been overlooked as a mechanism of compaction because it has been poorly understood and techniques for observing it are not commonly used. I have used scanned cathodoluminescence (CL) to quantify brittle deformation of quartz grains, in sandstones undergoing burial and cataclasis. Sandstone samples of different ages and compositions, taken from two basins with contrasting burial histories, are used to examine the interaction viii between brittle deformation and quartz cementation in burial compaction. Trends of increasing deformation by microfracturing with maximum burial depth are observed in both the lithic-rich Frio Formation from the Gulf of Mexico basin and in the quartz-rich Mount Simon Formation of the Illinois basin. Combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures allows for the calculation of the amount that brittle deformation influences compaction (i.e. porosity loss). For the Frio, 0.12 to 8.37% of initial porosity is lost due to cementation related to brittle deformation, whereas the values for the Mount Simon lie between 0.25 and 2.16%. Diagenetic forward models are constructed for each formation using petrographic modal analysis and burial history information to determine the depth of quartz cement commencement as an influential factor affecting brittle grain deformation. Most fracturing probably occurred prior to the precipitation of > 2% quartz cement. Commencement of quartz cementation at shallow depths combined with slower burial rates resulted in less brittle deformation in the Mount Simon compared with the Frio, where sandstones underwent rapid burial and quartz cementation began at greater depths. Cataclastic sandstones within the Pine Mountain Overthrust, eastern Kentucky, show more extreme porosity reduction by fracturing and cementation than normally compacted sandstones. In contrast to normal burial compaction, cataclasis and cementation within the cataclasites occurred over several discrete episodes as evidenced by cross-cutting relationships of fractures and cement. Quantitative data on the distribution of inter- and intragranular quartz cement within cataclasized sandstones combined with CL observations show that the timing of deformation is in agreement with published dates of fault movement.Item Petrophysical study of the Glorieta-Clearfork dolomite in the Monahans Field, Ward County, Texas(Texas Tech University, 1992-12) Saha, SouvickCalculation of a reservoir's water saturation using the Archie equation requires the values for cementation exponent (m) and saturation exponent (n). Determination of these two parameters, particularly in carbonate reservoirs, is often difficult. Recently a new method (CAPE) for estimating m and n has been proposed by Maute et al. (1992). In the CAPE (Core Archie Parameter Estimation) method, m and n are determined by minimizing the error between laboratory derived water saturation (Sw (core)) and water saturation calculated by the Archie equation (Sw(Archie)). Because core data are often unavailable, the author substituted dielectric water saturation (Sxo(dielectric)) for core-derived water saturation to determine m and n, and applied the technique to the Glorieta-Clearfork dolomites in the Monahans field, Ward County, Texas. The Permian (Leonardian) Glorieta-Clearfork dolomites in the Monahans field represents an upward-shoaling carbonate platform sequence. The predominant rock type is dolostone and the major mineral constituents are dolomite and anhydrite. Petrographic analysis reveals mainly intercrystalline/intergranular pore geometry with minor vuggy/moldic porosity. In this study the author applied three techniques: (1) non-linear, (2) linear, and (3) m-porosity transform to determine m and n values that minimize the error (errorfunction) between Sxo (dielectric) and Sxo (Archie). Two mporosity transforms were established, but the transform that represented the majority of the data (85%) was used to derive m values. Using data from the Glorieta-Clearfork dolomite in the Monahans field, the non-linear method resulted in the minimum error between Sxo (dielectric) and Sxo (Archie). The m and n values determined by the non-linear and linear methods probably do not represent physical rock characteristics but are only values that minimize the error functions. In contrast, m and n values determined by the mporosity transform method should represent physical attributes of reservoirs such as pore geometry or wettability. In order to further reduce the error between Sxo (dielectric) and Sxo (Archie), m and n were approximated by mathematical functions (polynomial and Fourier series) to model the vertical variation of m and n in the reservoir (variable m and n method). This variable m and n method based on a Fourier series resulted in the greatest errorreduction when compared to the non-linear method. After determining m and n values that result in the minimum error between Sxo (dielectric) and Sxo (Archie), these values can then be used to calculate the water saturation in the uninvaded zone (Sw).Item Study of petrography and internal structures in calcretes of West Texas and New Mexico(Texas Tech University, 1986-05) Chitale, Jayashree Dattatraya.