Organic Geochemistry of Sediments of Deep Gulf of Mexico Basin
Authors: Fang J. , Sassen R. , Chinn E.W. 
Published: October 2008

Rock-Eval pyrolysis of 687 core samples from the Mississippi submarine fan and Orca and Pigmy Basins in northern Gulf of Mexico has revealed that TOC and hydrogen index are, respectively, 0.82% and 103 mg HC/g TOC. Tmax values average 425° C. It is concluded that the sediments of Pleistocene and Holocene age in the deep Gulf of Mexico Basin are organic-lean, thermally immature, and are not obvious analogs to the source environments in the geologic past that gave rise to Gulf Coast crude oil.

INTROPUCTION

Despite the immense volumes of hydrocarbon production from the Gulf Coast Tertiary, especially from the Miocene and younger reservoirs, the origin of crude oils has not been thoroughly documented1,2,3. study on the origin of the crude oils in the northern Gulf of Mexico has been the subject of many efforts in recent years. It was believed that oil and gas were generated and expelled from shallow water shales "soon after deposition and burial" 4,5. Clark and Rouse4suggested that all environment, structural, and stratigraphy elements necessary the generation and entrapment of hydrocarbons were present within the Cenozoic deltas of the Louisiana Gulf Coast and constitute a closed system which is responsible for the large hydrocarbon accumulations in the Cenozoic oil fields of the Louisiana Gulf Coast. These ideas were based on geological grounds and absent of research on source rock potential and thermal maturity. Many organic geochemists accept that crude oil andthermogenic gas in reservoirs of the GUlf Coast Salt Basins are the result of vertical migration from deep source rocks 6,7,8,9,10. Nunn and sassen 10 suggested that the generation of crude oil and thermogenic gas take place at great depth in sediments of the Gulf Coast Basin. The enormous accumulation of hydrocarbons in the basin resulted from vertical migration from deeply buried Mesozoic source rocks that are not usually encountered by drilling.

The purpose of the present study' is (1). to determine kerogen type, abundance and thermal maturity of organic matter in samples coring at DSDP (Deep Sea Drilling Project) Leg 96 from site 614A through 624, (2) to assess the generative potential for crude oil of the sediments, given deeper burial and higher levels of thermal maturity.

The study area is located in the Gulf of Mexico Basin (Fig. 1). Cores from nine DSDP Leg 96 Mississippi Fan sites and two intraslope basin sites provided samples for pyrolysis analysis. Samples come from middle fan sites 617, 620, 621 and 622, lower fan sites 614, 615, 623 and 624, and the two intraslope basin sites (Sites 618 and 619) 11 (Table 1). The present study is based on the pyrolysis results of the 687 samples from these cores.

SAMPLES AND METHOPS

Sediment samples were collected on the Glomar Challenger during DSDP Leg 96 12. The cores used in this study were taken in water depths of 2495 to 3314 m, and from the youngest fan lobe and the two intraslope basins (Fig. 1), of Pleistocene age, in the Gulf of Mexico Basin. Amount cored varied considerably, ranging from 75 m to 421.3 m (Table 1).