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

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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