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Source Rock in the Lower Tertiary and Cretaceous, Deep-Water Gulf of Mexico
Bruce E. Wagner, Zvi Sofer, Brenda L. Claxton
The MC-84 (King) well was drilled in the deep-water Gulf of Mexico in 1993, in Mississippi Canyon Block 84 in a water depth of 5,149 ft. This well drilled an anticlinal feature. The well penetrated an Upper Cretaceous section and crossed the Middle Cretaceous Unconformity with final total depth in the lower Cenomanian.
Numerous sidewall cores were taken throughout the Lower Tertiary and Cretaceous. Six of the sidewall cores (from 14,230 to 15,170 ft subsea) are organic rich and contain Type II oil-prone kerogen (TOC values from 2.6 to 5.2% and hydrogen indices from 360 to 543 ppm). The Lower Tertiary through Lower Cenomanian section is thermally immature for oil generation, on the basis of biomarker ratios and vitrinite reflectance measurements. Organic extracts from cores in the Cretaceous section had biomarker characteristics similar to oil recovered from the Miocene in the MC-84 well. The oil was generated from a similar but more mature source rock, probably of Early Cretaceous age.
Prior to drilling, thermal maturities and temperatures were calculated using a commercial modeling program using heat flow values and lithologies from surrounding wells, which were compared to calculations using post-drilling well data (heat flow values and lithologies). These results were also compared to measured thermal maturities from sidewall core samples. Although modeling calculations, measured temperatures, and thermal maturities are not in perfect agreement, they all indicate that the penetrated section is immature for oil generation.
Results of thermal modeling indicate that the only section thermally mature for oil generation is in the lower portion of the Lower Cretaceous, below the total depth of the well. The model also indicates that the organic-rich section equivalent to that penetrated by the MC-84 well could be mature farther to the north, where water depths are shallower, overburden thickness is greater, and heat flow is higher.
Late Tertiary sediment loading in this area, primarily during the Miocene, is probably the driving mechanism for hydrocarbon generation from the Cretaceous (and possibly the Lower Tertiary) potential source rocks. This offers a favorable geological setting for capturing hydrocarbons because reservoirs and traps associated with Miocene deposition and subsequent loading-induced salt movement had formed prior to the onset of oil generation and migration.
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