ABSTRACT: Fluid Pumping and Natural Hydrofracturing in the Austin Chalk: Geochemical and Fabric Constraints

CORBETT, K. P., Marathon Oil Company, Littleton, CO, W. LAMB and D. V. WILTSCHKO, Texas A&M University, College Station, TX

Natural hydrofracturing occurred in the Austin Chalk as a result of abnormal fluid pressure build-up driven by dewatering of the underlying Eagleford Shale. Primary evidence for the dewatering mechanism is derived from field observations of vein fabrics, the geometry and morphology of veins and fractures in core samples, the isotopic and trace element composition of vein filling calcite, the stoichometry of vein filling clay minerals, and pore fluid pressure estimates obtained from fluid inclusions geothermometry coupled with vitrinite temperature estimates.

Austin Chalk veins typically contain two primary minerals. Mixed-layer smectite-illite clay lines the vein walls and was the first species deposited from solution. Subhedral, medium- to coarse-grained calcite was deposited next. The clay minerals on average have a 2:1 smectite:illite ratio. Clay minerals constitute approximately 20% of the vein fill by volume, an order of magnitude greater abundance than in the surrounding chalk matrix. Vein calcite is depleted in Sr, Fe, and Mn and enriched in Mg with respect to the chalk matrix. This relationship is at odds with the expected trace element composition if the vein calcite were derived from dissolution of adjacent chalk. Fluid inclusion microthermometry was done on outcrop vein calcite samples from San Antonio and Waco. The San Antonio sample has a Th of 51.5 degrees C. The Waco samples have a mean Th of 67.5 degrees C and a maximum Th of 74 degrees C. Salinities for all samples are low with Tm equal to -4.4 degrees C and -3.9 degrees C for San Antonio and Waco samples respectively. Vitrinite reflectance values along the outcrop trend from Dallas, Austin, San Antonio and Del Rio range from 0.71 to 0.81. These reflectance values yield estimated temperatures of 86 degrees C to 97 degrees C using the Gulf Coast empirical "standard well curve." A conservative estimate of fluid pressure at the time of fluid inclusion formation is 230 MPa. This value is derived from using an isochore for the highest temperature Waco sample and the lowest vitrinite reflection temperature estimate. This value is in the range of estimated litho tatic overburden pressure for the Dallas to San Antonio outcrop trend.

Veins in both the outcrop and subsurface commonly display crack-seal texture. This texture is indicative of cyclic pore pressure build-up and subsequent dissipation during vein mineral deposition. Veins also commonly occur as anastomosing swarms up to 2 m wide, demonstrating more areally distributed as opposed to discrete failure. Finally, vein swarms have preferentially been the locus of faulting. Fault displacement occurred during vein formation, as indicated by progressive rotation of veins in the fault zones. The lack of crystal plastic deformation in fault zone calcite veins suggests either very low differential stress during fault displacement, or that the veins still contained a fluid phase during displacement which prevented mutual impingement of calcite crystals.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)