Many low matrix-porosity hydrocarbon reservoirs are productive because permeability is controlled by natural fractures. The understanding of basic fracture properties is critical in reducing geological risk and therefore reducing well costs and increasing well recovery. Unfortunately, neither geophysics nor borehole methods are, so far, accurate in the acquisition of key fracture attributes, such as density, porosity, spacing and conductivity. This study proposes a new protocol to predict key fracture characteristics of subsurface carbonate rocks and describes how using a relatively low-cost but rock-based method it is possible to obtain accurate geological information from rock samples to predict fracture attributes in nearby but unsampled areas. This methodology is based on the integration of observations of diagenetic fabrics and fracture analyses of carbonate rocks, using outcrops from the Lower Cretaceous Cupido Formation in the Monterrey Salient of the Sierra Madre Oriental, northeastern Mexico. Field observations and petrographic studies of crosscutting relations and fracture-fill mineralogy and texture distinguish six principal coupled fracturing-cementation events. Two fracture events named F1 and F2 are characterized by synkinematic calcite cement that predates D2 regional dolomitization. A third fracture event (F3) is characterized by synkinematic dolomite fill, contemporaneous with D2 dolomitization of host strata. The fourth event (F4) is characterized by synkinematic D3 baroque dolomite; this event postdates D2. The fifth fracture event (F5) is characterized by C3 synkinematic calcite, and postdates D3 dolomite. Finally, flexural slip faulting (F6) is characterized by C3t calcite, and postdates D3 dolomite. Carbon and oxygen stable isotopes were used to validate the paragenetic sequences proposed for the Cupido Formation rocks. The dolomite isotopic signatures are consistent with increasing precipitation temperatures for the various fracture cements, as is expected if fractures grew during progressive burial conditions. Three main groups of calcite cement can be differentiated isotopically. Late calcite cement may have precipitated from cool waters under shallow burial conditions, possibly during exhumation of the SMO. The development of the Structural Diagenetic Petrographic Study protocol, and its integration with geological, geophysical and engineering data, can be applied to oil fields in fractured carbonates such as those located in Mexico, to validate its applicability. / text

The effectiveness of CO₂ injection in the subsurface for storage and EOR are controlled by reservoir quality variation. This study determines the depositional processes and diagenetic alterations affecting reservoir quality of the Lower Tuscaloosa Formation at Cranfield Field. It also determines the origin, time and processes of the grain-coating chlorite and its impacts on reservoir quality. Moreover, by integrating depositional and diagenetic characteristics and by linking them to sequence stratigraphy, the distribution of reservoir quality, could be predicted within a sequence stratigraphic framework. The studied sandstones are composed of medium to coarse-grained, moderately sorted litharenite to sublitharenite with composition of Q76.1F0.4L23.5. Depositional environment of this formation in the Mississippi Interior Salt Basin is interpreted as incised-valley fluvial fill systems. The cross sections and maps at the field show trend of the sandy intervals within channels with a NW-SE paleocurrent direction. During burial of the sandstones, different digenetic alterations including compaction, dissolution, replacement and cementation by chlorite, quartz, carbonate, kaolinite, titanium oxides, pyrite and iron-oxide modified the porosity and permeability. Among these, formation of chlorite coats plays the most important role in reservoir quality. The well-formed, thick and continuous chlorite coatings in the coarser grain sandstones inhibited formation of quartz overgrowth, resulted in high porosity and permeability after deep burial; whereas the finer grain sandstones with the poorly-formed, thin and discontinuous chlorite coatings have been cemented by quartz. The optimum amount of chlorite to prevent formation of quartz overgrowths is 6% of rock volume. The chlorite coats are composed of two layers including the inner chlorite layer formed by transformation of the Fe-rich clay precursors (odinite) through mixed-layer clays (serpentine-chlorite) during early eodiagenesis and the outer layer formed by direct precipitation from pore waters through dissolution of ferromagnesian rock fragments during late eodiagenesis to early mesodiagenesis. In the context of the reservoir quality prediction within sequence stratigraphic framework, the late LST and early TST are suitable for deposition of chlorite precursor clays, which by progressive burial during diagenesis could be transformed to chlorite, and thus results in preserving original porosity and permeability in deep burial. / text

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In spite of the abundance of opening-mode fractures in the earth's upper crust, knowledge about their spatial arrangement remains limited. The spatial arrangement of fractures refers to the patterns of fracture positions in space. On one-dimensional analyses, fracture position can be obtained by combining fracture apertures, spacings, and their sequence along a one-dimensional scanline. Previous approaches failed to account for fracture position and fracture size, thus a new technique, normalized correlation count (NCC), was used to overcome these limitations. This technique was designed to distinguish random from non-random (fractal, inherited/imposed, periodically arranged fractures, or periodically arranged clusters) spatial arrangements of fractures. In addition, another method to quantify the attributes of microfractures in rock samples larger than a thin section was developed and used to quantify their spatial arrangements. NCC indicated that where statistically significant (non-random) clusters exist, large fractures are more clustered than small ones. Differential clustering according to fracture size was detected in data sets from different lithologies at outcrop and rock-sample scale, suggesting that this phenomenon is related to development of fracture systems as opposed to host rock lithology and scale. Fracture clusters with power-law variation of spatial correlation with length scale are not strictly natural fractals because clusters occur in cascades at discrete values of length scale and not in a continuous fashion. Some statistically significant clusters with a power-law of spatial correlation are formed by smaller clusters with a power-law of spatial correlation that are also periodically arranged. Fractures from the Cupido Fm. in the Monterrey salient were grouped in three categories based on their trace morphology, cement composition, and timing of fracture cements with respect to fracture opening. Fractures at outcrop scale in two of the categories exhibit low percentages of synkinematic cement and random arrangements, whereas fractures in the remaining category exhibit large amounts of synkinematic cement and periodically arranged clusters. An evolutionary model of fracture development based on subcritical propagation is proposed. This model suggests that mechanical layering increases during cluster development, explaining the non-random clustering within interclustering domains at outcrop scale and implies that cluster spacing increases with mechanical layering but decreases during evolution towards cluster saturation. / text

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Deep-water clastic sediments of the lower Paleozoic continental margin in the Quebec City area are piled up in imbricated thrust sheets and nappes constituting the External Domain of the Northern Appalachians. The youngest sediments (Middle Ordovician Citadelle and Quebec City Formations) occu