(Hydro)fracturing and Fluid Flow in the Laramide Foreland-Fold-and-Thrust Belt of Eastern Mexico
Helga Ferket1, Rudy Swennen1, Salvador Ortuño-Arzate2 and François
1 Fysico-chemische Geologie, K.U.Leuven, Celestijnenlaan 200C, B-3001 Heverlee, Belgium ([email protected]; [email protected])
2Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, Col. San Bartolo Atepehuacán, Apartado postal 14-805, 07730 México, D.F., México ([email protected])
3Institut Français du Pétrole, 1 et 4 Avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France ([email protected])
The Cordoba Platform in eastern Mexico belongs to the eastern border of the North American Cordillera, which constitutes a Laramide foreland-fold-and-thrust belt (FFTB). It hosts a number of large oil fields in the tectonic front area. Reservoir-equivalents (M. to U. Cretaceous platform carbonates) crop out in the west and are buried in the east, where they occur below a Tertiary-Quaternary foreland basin. A structural analysis (vein morphology, orientation and relative timing, see also Ferket et al., 2004) and diagenetic study (based on classical petrography, cathodoluminescence (CL), scanning electron microscopy (SEM), stable isotopes, Sr-isotopes and fluid inclusion (FI) microthermometry, see also Ferket et al., 2003) has been performed on veins in this area. Crosscutting relationships of these veins with different sets of stylolites allow to fit the paragenetic sequence on the deformation history. Bed-parallel stylolites (BPS) reflect vertical compaction and thus relate to the burial episode, while layer-parallel-shortening stylolites (LPSS) or tectonic stylolites reflect horizontal compaction and indicate the onset of tectonic compaction. The different vein generations are then placed in the kinematic context and related to inferred paleostress conditions. The aim of this study is to gain some insights into the deformation – fluid flow history during FFTB-development.
Special attention was paid to the development of ‘breccia-veins’, which are frequently reported from FFTB-settings worldwide (e.g. Swennen et al., 2003) and which are often interpreted as an indication for paleohydrofracturing because of their morphology. The study area forms an excellent case for this type of veins, since they occur here in different tectonic stages. Paleohydrofracturing is difficult to prove directly without suitable fluid inclusion measurements, but indirect evidence for paleohydrofracturing is given based on morphology, paleostress evolution and kinematic context.
From Upper Cretaceous to Paleocene the study area evolved from stable platform to Laramide compressional belt. During the early burial stage (pre- to syn-BPS veins), multiple crack-seal/crack-jump veins (sometimes with breccia-vein outline) developed, which testify of a cyclic stress field and which could be related to foreland flexuring. During this stage, host-rock buffered fluids predominate. This is based on a similar mineralogy, CL-pattern and isotopic compositions in the veins as in the matrix and on the presence of monophase aquous fluid inclusions typical for temperatures below 50°C.
Due to increasing tectonic stress, a shift between horizontal and vertical stress as main compressive stress occurred, leading to a period of lowered differential stress. During this stage, less-well-oriented veins and breccia-veins formed. An increase in fluid pressure is required to explain extension fracturing (hydrofracturing). Post-BPS / pre- to syn-LPSS veins should relate to the prefolding compressional stage (e.g. Storti and Salvini, 2001) and are interpreted to develop in a caterpillar-type evolution of compression with fluid expulsion (hydrofracturing) and LPSS-formation. This early compressional phase shows still host-rock buffered characteristics, but a slight increase in temperature (based on depleted 18O-values and FI homogenisation temperatures between 40 and 55°C).
LPS-deformation prograded through the platform from west to east (de Cserna, 1989). LPS-parallel structures have the potential to be opened during folding of the strata and new fractures may form as extrados-structures in anticlinal hinges. Consequently, vertical fluid migration pathways may be opened allowing a larger vertical fluid circulation. This is corroborated by the frequent observation of cemented LPSS-planes in these settings and by the observed evolution in geochemistry of the vein-filling phases. Early post-LPSS phases indicate more elevated temperatures (more depleted 18O and FI homogenisation temperatures between 50 and 65°C) and varying mineralogy (saddle dolomite, fluorite,…).
Due to increased differential stress and confining pressure during the main compressive phase, shear fractures tend to develop. Post-LPSS subhorizontal veins in the study area are interpreted in relation to bedding-parallel shear and/or to the migration of thrust sheets. The infilling phases point towards host-rock buffered, cooler fluids (based on stable isotope composition). However, these veins are often reused by later karst-processes (see below).
The emerged thrust sheets in the western part of the study area are subjected to erosion. Moreover, the area is affected by an extensional phase during Oligocene. Consequently, a telogenetic karst developed with large dissolution and terra rossa infills and with dissolution-enlarged fractures, breccia-formation and geopetal infill of vadose silt and drusy calcite cement. Often, older vein generations are reactivated. The infilling phases show a trend towards meteoric water and karst-related processes (FI final melt temperatures around 0°C and depleted 13C-values trending towards or similar to the signature of recent speleothem cement).
The eastern part of the study area is buried below a subsiding foreland basin from Eocene to Quaternary. Before onlapping, some paleokarst episodes affected the tectonic front area. An intense, multiple brecciation affected dolomite strata just above a major thrust plane. These breccias may reflect the damage zone of the fault. Calcite spherulites with
13C-values similar to oil and with a serrated, concentric CL-zonation fill in these breccias and are interpreted as bacterial precipitates in relation to oxidation of oil. Coincidence of several geologic processes (thrust emplacement with brecciation, local emersion with karst and oil maturation and migration) led to precipitation of these peculiar cements. The latter situation also explains why the studied well is not oil-producing.
In conclusion, we can state that coupling fluid flow history to the deformation history by a combined microstructural – diagenetic study of veins leads to a dynamic scenario of fracturing and fluid flow during FFTB-evolution.
De Cserna, Z., 1989. An outline of the Geology of Mexico. In: Bally, A.W. and Palmer, A.R. (eds.), The Geology of North America - An overview. Boulder, Colorado, Geological Society of America, The Geology of North America., Volume A, 233-264.
Ferket, H., Swennen, R., Ortuño-Arzate, S. and Roure, F., 2003. Reconstruction of the fluid flow history during Laramide foreland fold and thrust belt development in eastern Mexico: cathodoluminescence and 18O-13C isotope trends of calcite-cemented fractures.
Ferket, H., Swennen, R., Ortuño-Arzate, S., Cacas, M.C. and Roure, F., 2004. Vein formation in Cretaceous carbonates in the Laramide Foreland Fold-and-Thrust Belt (FFTB) of eastern Mexico. AAPG Post-Hedberg Conference volume (Mondello – Sicily, 2002), in press.
Storti, F. and Salvini, F., 2001. The evolution of a model trap in the central Apennines, Italy: Fracture patterns, fault reactivation and development of cataclastic rocks in carbonates at the Narni Anticline. Journal of Petroleum Geology 24, 171-190.
Swennen, R., Ferket, H., Benchilla, L., Ellam, R. and SUBTRAP-team, 2003. Fluid flow and diagenesis in carbonate dominated Foreland-Fold-and-Thrust Belts: petrographic inferences from field studies of late-diagenetic fabrics in Albania, Belgium, Canada, Mexico and Pakistan. Journal of Geochemical Exploration 78-79, 481-485.
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