The Nubian Sandstone Formation in the Central Graben Area (Libya - Sirte Basin): Stratigraphic Framework and Sedimentary Evolution

Minervini, Matteo ¹; Serafini, Giuseppe ¹; Betikh, Abdulmonem Salem ²
(1)E&P, Eni S.p.A., San Donato Milanese, Italy. (2) National Oil Corporation, Tripoli, Libyan Arab Jamahiriya.

The Sirte Basin is composed by several narrow grabens confining thick accumulations of fluvio-deltaic to lacustrine/marginal-marine deposits. The study area is located in the Central Graben, corresponding to the eastern sector of the Sirte Basin affected by strong subsidence during the Cretaceous. The sedimentary succession, belonging to the Nubian depositional cycle, unconformably overlies older basement rocks and it is mainly subdivided into three informal stratigraphic units: Lower Nubian Sandstone (LNS: Barremian), Varicoloured Shale (VS: Early Aptian-lower part of Late Aptian), Upper Nubian Sandstone (UNS: lower part of Late Aptian - Early Albian).

This tectono-sedimentary cycle was terminated by the Top Nubian Unconformity (TNU), overlied by the post-Nubian depositional cycle (Reworked Sequence, Rakb Formation). The TNU was the stratigraphic response to a marked basin modification, associated to an incipient rifting phase. In addition, this tectonic event generated a full set of volcanic rocks (intrusive to effusive), belonging to the Volcanic Unit (VU).

The integration of a full bottom core dataset, well log correlation panels calibrated with 2D seismic and qualitative facies maps led to the identification of an alluvial-lacustrine depositional setting apparently affected by a marginal-marine influence both in the LNS as well as in the VS and UNS.

A detailed physical stratigraphic framework was built through the identification of key depositional surfaces suggesting a cyclical behaviour of the sedimentary record. Actually, this stratigraphic interval was characterized by an overall forestepping trend with the proximal depositional elements (e.g. fluvio-deltaic sedimentary bodies) progressively prograding onto distal sectors (lacustrine to marginal-marine deposits). This trend was modulated by higher-frequency ciclicity with rapid and sharp backstepping of finer-grained levels behaving as permeability baffles and/or barriers.

Combining our understanding of UNS stratigraphic/sedimentary evolution and its relationship with the VU, enabled us to make predictions about the depositional architecture in the Central Graben Area during the Cretaceous and thus the potential distribution of the reservoir-like facies.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.