25th International Meeting on Organic Geochemistry IMOG 2011

P-078
The record of the early Aptian global oceanic event OAE1a in
Goraa-Hammam Biadha Basin (Northwestern Tunisia)
Soumaya Abbassi 1,2 , Habib Belayouni 2 , Moncef Saidi 3
1 Macquarie University, Sydney, Australia, 2 Faculty of Sciences of Tunis, Tunis, Tunisia, 3 Entrprise
Tunisienne des Activites Petrolieres, Tunis, Tunisia (corresponding
author:soumaya.abbassi@students.mq.edu.au)
the organic geochemical study, based on both Rock-
Eval and total lipidic extracts data, of two outcropping
sections across the Goraa-Hammam Biadha Basin
have led to point out the following results and
conclusions: (1) the Barremian-Aptian series are
made up of pelagic successsion which is
characterized by cyclically arranged marls and
argillaceous/massive limestones with the occurrence
of some thin black shale horizons; (2) the most
organically rich materials appear to be associated
with massive limestones and with black shale levels;
(3)The Barremian-Aptian series exhibit significant
thickness variation along a NW-SE axis. The highest
thickness (up to 90 meters) is reached towards the
Triassic massif of Arkou-Fedj El Hdoum, while to the
NW, close to the Triassic Massif of Thibar, series are
only 30 meters thick; (4) along a NW-SE axis
addressed across the investigated basin, the Total
Organic Carbon (TOC) average amounts decrease
from 2.5% to 1.72%, a TOC amount as high as 5.8%
has been registered only at the north western part of
the study basin; (5) the total pyrolitic yields (S1+S2)
are also found to decrease from 8.27 mg HC/ g rock
in the NW to 1.69 mg HC/ g rock in the SE, thus
confirming that the Barremian-Aptian series have
good potential oil and gas prone source rocks (HI
indices values for immature levels are up to 435 mg
HC/ g TOC ), with a singular and uncommon
petroleum potential differentiationalong a NW-SE
axis; (6) in term of thermal maturity evolution
(deduced from the Tmax measured values), the late
Barremian-Aptian organic matter displays a variable
and an increased rank of thermal maturity towards the
same direction of the thickening trend (Immature to
early mature source rock: 438°C to the NW and over
mature source rock: Tmax: 495°C to the SE). The
occurrence of these sediments reflects restricted
conditions leading to the accumulation of nutrients
and mainly to the development of anoxia. Accordingly,
such anoxic environment within the basin can be
explained by the existence of physical barrier which
prohibited the changes with the open sea. In addition,
The Goraa-Hammam Biadha basin effectively acted
as a half-graben structure tilted to the south-east
(Fig.1) which had been initiated under a general
extensional regime. Within such a half-graben basin,
the present day Triassic massifs of Thibar to the NW
and Fedj El Hdoum to the SE, acted as active
extensional NE-SW trending faults through which the
rising up of Triassic sediments has induced the
initiation of uplifted structures, and the formation of a
semi-enclosed basin, favourable to the accumulation
and preservation of high amounts of organic matter
associated to typical syn-rift deposits.
Fig1. Lateral correlation between the two studied sections along
a NW-SE axis.
REFERENCES
Langford F. F., and Blanc-Valleron M. M. (1990) Interpreting rock-Eval pyrolysis
data using graphs pyrolyzable hydrocarbons vs.total organic carbon. Am Assoc
Pet Geol Bull 74, 799-804.
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