A new method is presented here for petrophysical interpretation of heterogeneous carbonates using well logs and core data. Developing this new method was necessary because conventional evaluation methods tend to yield inaccurate predictions of pore connectivity and permeability in the studied field. Difficulties in the petrophysical evaluation of this field are related to shoulder-bed effects, presence of non-connected porosity, rock layers that are thinner than the vertical resolution of well-logging tools, and the effect of oil-base mud (OBM) invasion in the measurements. These problems give rise to uncommon measurements and rock properties, such as: (a) reservoir units contained within thinly bedded and laminated sequences, (b) very high apparent resistivity readings in the oil-bearing zone, (c) separation of apparent resistivity logs with different depths of investigation, (d) complex unimodal and bimodal transverse relaxation distributions of nuclear magnetic resonance (NMR) measurements, (e) reservoir units having total porosity of 0.02 to 0.26 and permeability between 0.001mD to 4.2D, (f) significant differences between total and sonic porosity, and (g) low and constant gamma-ray values. The interpretation method introduced in this thesis is based on the detection of layer boundaries and rock types from high-resolution well logs and on the estimation of layer-by-layer properties using numerical simulation of resistivity, nuclear, and NMR logs. Layer properties were iteratively adjusted until the available well logs were reproduced by numerical simulations. This method honors the reservoir geology and physics of the measurements while adjusting the layer properties; it reduces shoulder-bed effects on well logs, especially across thinly bedded and laminated sequences, thereby yielding improved estimates of interconnected porosity and permeability in rocks that have null mobile water saturation and that were invaded with OBM. Additionally, dynamic simulations of OBM invasion in free-water depth intervals were necessary to estimate permeability. It is found that NMR transverse relaxation measurements are effective for determining rock and fluid properties but are unreliable in the accurate calculation of porosity and permeability in thinly bedded and highly laminated depth sections. In addition, this thesis shows that low resistivity values are associated with the presence of microporosity, and high resistivity values are associated with the presence of interconnected and vuggy porosity. In some layers, a fraction of the vuggy porosity is associated with isolated pores, which does not contribute to fluid flow. An integrated evaluation using multiple measurements, including sonic logs, is therefore necessary to detect isolated porosity. After the correction and simulation, results show, on average, a 34% improvement between estimated and core-measured permeability. Closer agreement was not possible because of limitations in tool resolution and difficulty in obtaining a precise depth match between core and well-log measurements. / text

Two hydrogeologic models explore reflux dolomitization using two outcrop datasets at different scales to constrain transient boundary conditions and heterogeneous petrophysical properties. A platform-scale petrophysical model of the Permian San Andres Formation was built from outcrop and subsurface data following a reservoir modeling approach that preserves outcrop heterogeneity and incorporates a sequence stratigraphic framework. This model was used as input for hydrogeological simulations of hypersaline fluid flow and solute transport during the accumulation and compaction of the platform. Boundary conditions change over time, as relative sealevel fluctuations drive sedimentation, depositional environment migration, topographic gradients, and location, size and salinity of the brine source. The potential volume and distribution of dolomite formed is inferred by a magnesium mass-balance. The composite result of reflux events at various orders of stratigraphic hierarchy is a complex dolomite pattern that resembles that observed on San Andres outcrops. Dolostone bodies across the platform may be generated by different combinations of favorable conditions, including proximity to the brine source, zones of higher permeability, permeability contrasts, and latent reflux. A meter-scale reactive transport model of the Albian Upper Glen Rose Formation simulates deposition of three high-frequency cycles punctuated by three brine reflux events. The simulator determines flow, solute and reactive transport along the flow paths, revealing the spatial and temporal distribution of calcite dissolution, and precipitation of dolomite and sulfate. The model recreates fully and partially dolomitized cycles within the time and lithological constrains on Glen Rose outcrops. Our results show that the distribution of dolomite within a high-frequency cycle may be the net result of intercycle processes, whereby dolomitizing fluids sourced from younger cycles flow across stratigraphically significant boundaries. We also show that variations in dolomite abundance and the unfulfilled dolomitization potential control the contemporaneous propagation of multiple dolomite fronts and the coalescence of discrete dolomite bodies. Results show that reflux is an effective and efficient mechanism to dolomitize carbonate formations that progresses simultaneously with sediment accumulation. Dolomitization is the cumulative result of many short-lived reflux events, sourced in different locations and times, and amalgamation of successive dolostone bodies. This model contrasts with previous studies that approached dolomitization of a carbonate platform as a discrete reflux event and current interpretations that relate dolomite bodies to their most immediate stratigraphic surfaces. / text