Source Rock Potential of Shallow-Water Continental Evaporites: Investigation from Holocene-Pleistocene Evaporite-Carbonate Sequences of Salt Flat Playa, West Texas-New Mexico

Mahbub Hussain

Source rock potential of the shallow-water evaporite-carbonate sequences at the Salt Flat Playa, evaluated on the basis of total organic carbon (TOC), nature of the organic carbon, preservation, and generation potential assessed by Rock-Eval pyrolysis, suggests similar sequences in the geological record should serve as moderate to good-quality source beds. Average TOC content of the sediments is about 0.57% but may be 5.0% or more in gypsum-rich sediments. Organic matter in gypsum-rich horizons is dominantly of algal origin, showing a strong n-C17 alkane peak in gas chromatograms and a carbon preference index value close to unity (1.04). Genetic potential value ranges from a low 1.85 kg/ton for carbonate-rich beds to a high of 5.42 kg/ton for gypsum-rich beds, characteriz ng gypsum-rich beds as an excellent potential source bed. Hydrogen index value is generally high, ranging from a low of 300 mg HC/g C_org for carbonate-rich beds to a high of 888 mg HC/g C_org for gypsum-rich beds.

Sedimentological evidence indicates periodic anoxicity in the Salt Flat brine lake during evaporite deposition. This was caused during wetter intervals when inflowing runoff water rested on denser brine, forming a halocline. Brine stratification inhibited free circulation of oxygen in the lake bottom, preserving the accumulated organic matter. The present-day Salt Flat Playa is a hydrologically closed depression, located in an arid region. The inflowing ground water is highly saline (up to 350% TDS) and oxygen depleted, eliminating the possibility of postdepositional diagenetic degradation of the organic matter.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.