Temperature, Pressure, and Fluid Composition Conditions of Fracture Cementation in the Brooks Range Fold-Thrust Belt, Arctic National Wildlife Refuge, Alaska
T.M. Parris1, R.C. Burruss2,
1Petro-Fluid Solutions, LLC, 608 North Sheridan Avenue, Loveland, CO 80537
2U.S. Geological Survey, M.S. 956, 12201 Sunrise Valley Drive, Reston, VA 20192
Analysis of quartz cemented fractures in thrusted Triassic and Jurassic siliciclastic rocks has provided critical insights into the timing of gas generation and migration for the eastern Brooks Range fold-thrust belt in the Arctic National Wildlife Refuge, Alaska. Fracture paragenesis, and the temperature, pressure, and fluid composition conditions of fracture cementation were reconstructed using petrographic, scanning electron microscopy and cathodoluminescence (SEM-CL), fluid inclusion, electron probe (EPMA), and oxygen isotope (conventional fluorination and secondary ion mass spectrometry) methods.
Most of the fractures appear to be tension features based on the absence of apparent offset. In addition to quartz, the fractures contain smaller amounts of ankerite and chlorite cement. Chlorite is present along the boundary between the fracture and wall-rock and ankerite fills the interstices between quartz crystals. In one sample, collected from a normal fault zone that postdates the thrust faults, ankerite predates quartz.
The morphology and textures of the quartz provide a record of cement growth before, during, and after deformation. Straight sub- to euhedral quartz crystals (up to 20 mm long) oriented at a high angle with respect to the fracture wall, and containing no internal crack-seal textures are interpreted to have grown relatively uninterrupted into fracture porosity following crack opening. Composite fracture-fills, containing enclaves of wall-rock in the fracture cement, represent multiple crack opening events followed by cementation. Some elongated quartz crystals (i.e. quartz fibers) are tilted away from the normal to the fracture wall or show a curved habit. The textures imply quartz growth before or during deformation, or both. The tilted and curved fibers contain inclusion-rich cores that contain hundreds of sub-parallel fluid inclusion trails oriented at a high angle with respect to the principal growth direction along the c-axis. In SEM-CL imagery, the fluid inclusion trails coincide with hundreds of micron-scale alternating dark and bright cathodoluminescence bands. The inclusion trails are inferred to represent crack-seal zones and the trails imply that the quartz fibers developed over many crack opening and cementing cycles. SEM-CL also reveals healed microcracks as dark stringers that cross-cut primary growth textures (e.g. sector boundaries and oscillatory zoning), and are further evidence of post-cementation deformation.
Fluid inclusion homogenization temperatures (175°-250°C) in quartz (n= 109) and ankerite (n= 20) and temperature trends within fracture samples suggest that cements grew at 7 to 10 km depth (assume 0ºC surface temperature and geothermal gradient equal to 25ºC/km) during the transition from near maximum burial to the early stages of uplift. Systematic analysis of aqueous inclusions (n= 33) along individual crack-seal zones in the quartz fibers shows a homogenization temperature range of ~20ºC among the crack-seal zones along the length of the fiber. This implies a relatively small temperature variation over the course of crystal growth.
Some fluid inclusion assemblages consisted of aqueous inclusions accompanied by interpreted coeval CH4-rich (dry gas) inclusions. This inclusion assemblage, which suggests the presence of immiscible water and gas in fracture porosity during crystal growth, was used to model the pressure of cementation in the quartz fracture cement. The modeling was done by documenting low temperature (-150º to -70ºC) phase changes (CO2 final melting and CH4 homogenization) in the CH4-rich inclusions (n= 26). The phase changes can be used to define molar volume (V) and composition (mole fraction, X) in the CH4-CO2 system. These data, along with true trapping temperatures from coeval aqueous inclusions, were used in a modified Redlich-Kwong equation of state to calculate trapping pressure. Calculated pressures range from 1000 to 2100 bars, which exceeds hydrostatic pressure (<1000 bars, assumes 100 bars/km) at a burial depth of 7 to 10 km. Thus modeling suggests that fracture cements grew from overpressured pore fluids. In contrast, calculated pressures are close to hydrostatic for the sample from the normal fault zone. Remarkably, primary inclusion assemblages from a single crystal of quartz fracture cement suggest a temperature variation of 40ºC and pressure fluctuation of 100’s of bars.
Fracture pore fluid salinities, determined from aqueous final ice melting temperatures, equal 3.4 to 5.7 wt.% NaCl equivalent, which are slightly more saline than seawater (3.2 wt.% NaCl equivalent). The cemented fractures occur in stratigraphic units deposited in marine environments; therefore, it is plausible that fracture pore fluids were evolved marine waters derived from within the formation. Fluid oxygen isotopic composition was determined using oxygen isotope values from the fracture cements (n= 31), aqueous homogenization temperatures as a proxy for crystallization temperature, and the appropriate mineral-water fractionation equation. The resulting delta 18O water values range from 6.4 to 13.7 per mil. Since the fracture cements formed at temperatures exceeding 175ºC, where the fractionation factor is reduced, the isotopically enriched water values likely resulted from water-rock interaction.
EPMA analysis shows quartz fracture cements to contain high quantities of Al (1500 to 2500 ppm). Probe traverses show systematic variation in Al values between sector zones as defined with SEM-CL. The exception to the high Al values occurs in healed microcracks (<1 up to ~15 microns wide), where Al values are commonly an order of magnitude lower (< 1000 ppm). Fluid inclusion and SIMS analyses of microcrack trackways (areas where fracture crystals have repeatedly broken and healed, and up to 100’s of microns wide) imply that microcrack healing occurred in the presence of fluid having a composition similar to that from which the fracture cement originally precipitated. If so, the paucity of Al in the microcracks might reflect Al immobility during the crack healing process in which Si is derived from the quartz fracture cements, or it might suggest a slower rate of crack healing in which Al is not taken into quartz cement that heals the microcracks.
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