Content, Type, and Thermal Evolution of Organic Matter in Sediments from the Eastern Falkland Plateau, Deep Sea Drilling Project, Leg 71

Abstract

The quantity, type, and maturity of the organic matter in Recent through Upper Jurassic sediments from the Falkland Plateau, DSDP Site 511, have been determined. Sediments were investigated for their hydrocarbon potential by organic carbon and Rock-Eval pyrolysis. Kerogen concentrates were prepared and analyzed in reflected and transmitted light to determine vitrinite reflectance and maceral content. Total extractable organic compounds were analyzed for their elemental composition, and the fraction of the nonaromatic hydrocarbons was determined by capillary column gas chromatography and combined gas chromatography/mass spectrometry.Three main classes of organic matter can be determined at DSDP Site 511 by a qualitative and quantitative evaluation of microscopic and geochemical results. The Upper Jurassic to lower Albian black shales contain high amounts of organic matter of dominantly marine origin. The content of terrigenous organic matter increases at the base of the black shales, whereas the shallowest black shales near the Aptian/Albian boundary are transitional in composition, with increasing amounts of inert, partly oxidized organic matter which is the dominant component in all Albian through Tertiary sediments investigated.The organic matter in the black shales has a low level of maturity and has not yet reached the onset of thermal hydrocarbon generation. This is demonstrated by the low amounts of total extractable organic compounds, low percentages of hydrocarbons, and the pattern and composition of nonaromatic hydrocarbons. The observed reflectance of huminite and vitrinite particles (between 0.4% and 0.5% RQ at bottom-hole depth of 632 m) is consistent with this interpretation. Several geochemical parameters indicate, however, a rapid increase in the maturation of organic matter with depth of burial. This appears to result from the relatively high heat flow observed at Site 511. If we relate the level of maturation of the black shales at the bottom of Hole 511 to their present shallow depth of burial, they appear rather mature. On the basis of comparisons with other sedimentary basins of a known geothermal history, a somewhat higher paleotemperature gradient and/or additional overburden are required to give the observed maturity at shallow depth.A comparison with contemporaneous sediments of DSDP Site 361, Cape Basin, which was the basin adjacent and to the north of the Falkland Plateau during the early stages of the South Atlantic Ocean, demonstrates differences in sedimentological features and in the nature of sedimentary organic matter. We interpret these differences to be the result of the different geological settings for Sites 361 and 511.

C2-C8 Hydrocarbons in Sediments from Deep Sea Drilling Project Leg 75, Holes 530A, Angola Basin, and 532, Walvis Ridge

“…In the geochemical study of Site 511, DSDP Leg 71, it was shown (Schaefer et al, 1983) that intervals bearing Type-Ill kerogens contained 20-45% benzene (as a fraction of total C 6 hydrocarbons). On the other hand, in the black shales of" Site 511, which have a Type-II kerogen, the benzene proportion ranged from 4 to 8% at a maturity range of 0.4 to 0.5% vitrinite reflectance (von der Dick et al, 1983). This source-controlled effect is also seen in Figure 4.…”

Origin and Geochemistry of Cretaceous Deep-Sea Black Shales and Multicolored Claystones, with Emphasis on Deep Sea Drilling Project Site 530, Southern Angola Basin

“…Index map (inset) is modified from Sclater et al (1977). 5 to 7%, with values as high as 24%, in the lower Albian at Site 364 in the eastern Angola Basin. Average organic-carbon concentrations of 4 to 5% are typical of the Hauterivian-lower Aptian strata at Site 511 in the Argentine Basin (von der Dick et al, 1983), and less than 1% in the Aptian-Albian. The black-shale beds of Albian to Coniacian-Santonian age at Site 530 contain as much as 16.59b organic carbon with an overall average of 5.4% organic carbon as compared with less than 0.5% organic carbon in intercalated beds of red and green claystone (Meyers, Brassell, Hue, this volume).…”

Carbonaceous sediments and palaeoenvironment of the Cretaceous South Atlantic Ocean

“…However, at Site 511 sapropel deposition occurred during the early A!bian, whereas dysoxic clays were deposited from mid to late Albian time. This sequence represents the gradual ventilation of the basinalprovince of the Falkland Plateau (Deroo et al 1983;Von der Dick et al 1983).…”