Sequence Stratigraphy and Source Rock Characterization of Organic-Rich Shales Within the Jurassic Smackover Formation, Conecuh Embayment, Alabama, U.S.A.
The Upper Jurassic Smackover Formation is a prolific producer of hydrocarbons known throughout the U.S. Gulf Coast region, and typically consists of carbonate lime mudstones, ooid grainstones, microbial boundstones, and dolostones. Recent exploration efforts in the Conecuh Embayment of southwest Alabama revealed the presence of two black, siliciclastic shale layers containing abundant terrestrially derived organic matter within the Smackover Formation. The shale layers provide interesting insight into the sequence stratigraphy and paleoclimate of the Conecuh Embayment, and the source of the hydrocarbons accumulated there. The two shale layers reach a maximum thickness of 50 feet along the longitudinal axis of the embayment and pinch out along the rim of the embayment. X-ray diffraction shows the mineralogy of the shales to be dominated by clay minerals, with lesser amounts of quartz and carbonate. The dominant clay mineral found within the shales is illite and mixed layer illite-smectite. The dominant presence of illite is considered to be a result of diagenesis and related to the advanced thermal maturity of the samples. Detrital chlorite is also present within the samples along with minor amounts of potassium feldspar, pyrite, and kaolinite. Palynological analysis of the organic matter within them revealed the presence of several genera of ferns, mosses, and conifers suggesting a warm, humid climate during the late Jurassic. Source rock analysis of the shales shows insufficient total organic carbon (0.32 %) and poor quality Type-III kerogens for consideration as a source rock. Deposition of the Smackover Formation occurred during a third-order sea level rise during the late Jurassic. Deposition of the shale layers is interpreted to have occurred during relative falls in sea level allowing a greater influx of siliciclastics into the embayment. The relative sea level falls may be a result of imposing higher order sequences on the third-order sequence. In the model presented here the Smackover Formation is divided into three sequences with the Smackover carbonates forming the transgressive and highstand systems tracts, where the shales represent lowstand systems tracts. The results of this study provide a better understanding of the Jurassic petroleum system contained within the Conecuh Embayment.
Alabama, Conecuh Embayment, Little Cedar Creek, Sequence Stratigraphy, Smackover, Smackover Formation, Geology
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