Abstract: Diagenesis of Miocene Sandstones Offshore South Texas, USA: A Fluid Inclusion-Laser Ablation Stable Isotope Study

T. J. Primmer, E. A. Warren, N. H. Oxtoby, K. Schofield

The diagenesis of Miocene sandstones offshore South Texas has been studied using conventional petrographic techniques, fluid inclusion microthermometry and spatially resolved carbonate stable isotope analyses obtained using a laser ablation microsampling device. The sands are lithic in composition and contain an unusually high proportion of carbonate rock fragments. Principal cements identified were, quartz and calcite.

Microthermometry on aqueous and CH₄-CO₂ fluid inclusions indicate temperatures of precipitation of between 70 and 130°C for quartz cements and 110 and 130°C for calcite. Stable isotope analyses from different carbonate grain types (e.g. carbonate rock fragments, bioclasts or sparry pore filling cements) show that different components have different ^dgr¹⁸O and ^dgr¹³C compositions that reflect their diverse origin and temperatures of formation. The range in ^dgr¹⁸O composition for the sparry cements (-13.3 to -8.5^pmil PDB) confirms the relatively high temperatures of precipitation deduced from fluid inclusion studies and the calculated composition of water in equilibrium with the calcite (2.9 to 4.7^pmil SMOW) is simila to present day South Texas formation waters. ^dgr¹³C of carbonate cements ranges between -8 to -3.5 PDB indicating carbon input from migrating from thermogenic CO₂. These observations challenge existing interpretations of cementation in Tertiary sandstones

from the Gulf Coast and Gulf of Mexico which suggest that diagenesis was a steady-state process, related to subsidence and progressive increase in temperature. This study shows that quartz and carbonate cements are both late, high temperature cements that precipitated relatively rapidly in the presence of thermogenic gas.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994