Mineralogical and Geochemical Studies of Tertiary Sediments From the Eastern Niger Delta and Their Relationship to Petroleum Occurrence

Abstract

Mineralogical and chemical studies have been made of shale cuttings from the Akiri‐1 well in Oligocene — Miocene sediments of the Niger Delta. The < 2μm fraction from the 2,183 to 3,720m stratigraphic interval was analyzed by X‐ray diffraction. Kaolinite and chlorite are more abundant than other clay minerals. The illiteh mectite composition changes from a phase with 32% illite layers, to one with 82% illite layers over the 3,250 to 3,720m stratigraphic interval. Kaolinite decreased in shallow shales from 22 to 16% and increases to 29% in deep shales. Quartz increases in abundance with depth, while chlorite and plagioclase feldspar show no systematic trend with depth. Calcite is present in small amounts in all the samples. Variations in the bulk chemical composition of true shale with depth only show significant changes in the K2O and Fe2O3 content: other major chemical constituents show no systematic variations. The potassium appears to be derived from the pore water. The loss of smectite and abundance of Fe coincided with hydrocarbon occurrence. On the basis of the apparent systematic transformation of smectites to illites through mixed layers, the illite/smectite reference interval in the subsurface has been established: this interval coincided with hydrocarbon occurrence. Mineralogical and chemical changes observed are dependent on the pore system chemistry, the temperature, and possibly compaction effects.

Clay Sedimentology

“…In addition, the water expulsion could determine overpressuring and hydro-fracturing in the rocks submitted to clay diagenesis; however, the increase in fluid pressure could inhibit further dehydration until the water released through illitization migrates out of the rock (Colten-Bradley Surdam and Crossey 1987) 1987). Odigi (1986) reports in Tertiary sediments from the eastern Niger delta the synchronous loss of smectite, increase of iron and hydrocarbon occurrence, over the 3250 to 3720 meter-depth interval where illite-smectite varies from 32 to 82% i11ite layers. Burst (1969) and Perry and Hower (1972) proposed a shale-dewatering history marked by two or three stages succeeding one another during burial (Fig.15.14).…”

Clay‐mineral Studies of the Subsurface Niger Delta

“…A: Kaolinite + smectite + chlorite + mixed-layer illitehmectite, with minor illite; this assemblage is found in the Akri-SZ Well, and is similar to the assemblage at Akri-I described by Odigi (1986).…”

“…The role of clay minerals (and of smectite in particular) in identifying levels of oil generation and migration, have been discussed in studies of clay-mineral diagenesis by Weaver, 1960;Burst, 1969;Bruce, 1984;and Odigi, 1986. Correlation schemes have been established between stages of clay-mineral diagenesis and hydrocarbons formation by these Authors: little hydrocarbon generation occurs during early diagenesis, but the yield is higher during the meso-diagenetic stage.…”

“…The clay mineralogy of the Tertiary and Recent Niger Delta was investigated by Porrenga, 1966;Lambert-Aikhionbare and Shaw, 1982;Adeleye, 1978;Braide, 1983;Olorunfemi, 1983;and Odigi, 1986. Sediments of the Tertiary Akata and Agbada Formations were studied by Lambert-Aikhionbare and Shaw (1982), who concluded that the clays in the Akata Shales are detrital, with only minimal alteration attributed to the effects of burial diagenesis.…”

Palynological interpretation of the palaeoenvironments of Miocene strata of the well Igbomotoru-1, Niger delta

“…(1971, 19721, Merki (1972), Murat (1972, Oomkens (1974), Weber & Daukoru (19761, Asseez (1976), Evamy et a/. (1978), Petters (1979), Ejedawe (19811, Lambert-Aikhionbare (1982), Lambert-Aikhionbare & Shaw (1982), Ogbe (1982), Odigi (1987) and Bustin (1988). Despite the fact that exploration and exploitation have been going on in this petroliferous basin for several decades, there is relatively little published information on the palynology of the sediments.…”