ABSTRACT

The Stuard City Trend consists of a shelf-margin buildup of Middle Cretaceous carbonates, now buried to depths of 10,000 to 18,000 ft in south central Texas. Whole rock analyses of 92 samples from 16 wells along a 250 mile stike section show a 0¹⁸ range of -5.9 to -2.7 and a C¹³ range of -.7 to +5.1 relative to PDB. Oxygen isotopes become lighter toward the southwest. Whole rock values of c¹³ indicate that vadose diagenesis was not volumetrically important.

Individual cements were also analyzed. The two predominate cement sequences are: 1) fibrous crust, 2) inclusion-rich radiaxial, and 3) clear spar; or 1) fibrous crust, 2) inclusion-rich spar, 3) clear spar. Inclusion-rich radiaxial cements show 0¹⁸ values closely grouped about a mean of -2.6 PDB and C¹³ values between -29.1 to +3.2 PDB. Inclusion-rich spar cements likewise show 0¹⁸ values closely grouped about the mean of -2.8 PDB and C¹³ values ranging from -7.4 to +3.8 PDB. In contrast, the clear blocky spars exhibit a wider range of 0¹⁸ values, from -6.6 to -2.3 with a mean of -5.2 PDB; C¹³ values range from -5.5 to +4.5 PDB. No significant isotopic differences were observed in the final generation of clear blocky spar cement, between depths of 10,300 and 20,400 ft.

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

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