Abstract: Carbon and Oxygen Isotopic Evolution of Whole Rock and Cements from Stuart City Trend (Lower Cretaceous), South-Central Texas

D. R. Prezbindowski

The Stuart City trend in south-central Texas consists of a shelf-margin buildup of middle Cretaceous carbonate rocks, now buried to depths of 10,000 to 18,000 ft (3,000 to 5,500 m). Whole-rock analyses of 92 samples from 16 wells along a 250-mi (400 km) strike section show a ^dgrO¹⁸ range of -5.9 to -2.7 per mil and a ^dgrC¹³ range of -0.7 to +5.1 per mil (relative to PDB). Oxygen isotopes become lighter toward the southwest. Whole-rock values of ^dgrC¹³ indicate that vadose diagenesis was not volumetrically important.

Individual cements also were analyzed. The two predominant cement sequences are: (1) fibrous crust, inclusion-rich radiaxial, clear spar; or (2) fibrous crust, inclusion-rich spar, clear spar. Inclusion-rich radiaxial cements show ^dgrO¹⁸ values closely grouped around a mean of -2.6 per mil PDB and ^dgrC¹³ values between -29.1 and +3.2 per mil PDB. Inclusion-rich spar cements also show ^dgrO¹⁸ values closely grouped about the mean of -2.8 per mil PDB and ^dgrC¹³ values ranging from -7.4 to +3.8 per mil PDB. In contrast, the clear blocky spars exhibit a wider range of ^dgrO¹⁸ values, from -6.6 to -2.3 per mil, with a mean of -5.2 per mil PDB; ^dgrC¹³ values range from -5.5 to +4.5 per mil PDB. No significant isotopic dif erences were observed in the final generation of clear blocky spar cement, between depths of 10,300 and 20,400 ft (3,139 and 6,217 m).

The whole rock and cements are not in isotopic equilibrium with sampled formation fluids. Individual cements maintain an isotopic memory of successive cementation events during burial.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas