--> Abstract: Seismic Stratigraphy, Sedimentology and Reservoir Potential of Syn-rift Turbidites (Maragh Trough, Sirt Basin, Libya), by D. Klauser-Baumgartner and M. W. Kuehn; #90923 (1999)

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KLAUSER-BAUMGARTNER, DETLEF, and MARTIN WINTERSHALL KUEHN, Libya,Tripoli

Abstract: Seismic Stratigraphy, Sedimentology and Reservoir Potential of Syn-rift Turbidites (Maragh Trough, Sirt Basin, Libya)

The area of investigation covers Wintershall?s concession 96 (Fig. 3), which is located in the Maragh Trough, in the E of the Sirt Basin (Libya). Intracontinental rifting during the Triassic led to the formation of three NW-SE striking half grabens. Pre-rift Paleozoic sandstones on the top of hanging wall structures build giant oil reservoirs (As Sarah and Jakhira oil fields). They are unconformably overlain by syn-rift lake sediments which are the scope of this paper.

Seismic Stratigraphy of the Triassic Syn-Rift-Sequence

A seismic stratigraphic study (3D) reveals the large scale architecture of the syn-rift graben fill (Fig. 1), which is controlled mainly by tectonic activity. Syn-sedimentary rotation of fault blocks led to thick successions within the half-graben and relatively thin successions on the top of hanging wall structures. Seismic interpretation clearly shows the unconformity, which separates the pre-rift sandstones and the syn-rift shale/sandstone sequence. The unconformity is covered by a bituminous shale (up to 19 % TOC), which varies in thickness from 60 ft on the top of hanging wall structures to 340 ft in the half-graben.The bituminous shale was deposited during rift initiation phase, which is characterised by rapid basin subsidence. Sedimentation took place in an inland lake with possible contacts to the open sea, and the sediments act as source rock for the pre- and syn-rift sandstones.

At least five seismic parasequences follow on top of the bituminous shale in the Northernmost Half Graben. Progressively younger sequences onlap and downlap farther towards the SW onto the basal shale.The top of each seismic parasequence is interpreted along positive amplitude (reverse polarity, minimum phase). A seismic sequence, comprising the interval between two maxima (one period), has a thickness of around 40 ms (~300 ft).

The two uppermost parasequences are controlled by well data, which are only available on the top of the hanging wall structures.

Facies interpretation from conventional core data

Detailed conventional core studies of parasequence 5 reveal mostly fine grained sandstones of moderate sorting with weak normal grading. Sedimentary structures are dish structures, water escape pipes, groove casts and trace fossils. Deposition as turbidites is inferred. Thicknesses of single turbidites reach from several inches to a maximum of six feet.

The cored sequences contain Bouma divisions Ta to Te. Groove casts at the base of Ta divisions indicate variations of transport direction of 30°.

Sedimentological Model

Tectonic activity caused slope instability at the basin margin and triggered turbidite sedimentation in the basin.Turbidite megacycles build up five parasequences which can be resolved on seismic lines. Each parasequence starts with a coarsening upward trend of around 50 ft, when the first turbidites stacked on top of each other (Fig. 2). It continues with a fining upward trend of up to 300 ft, when the number of turbidites, entering the basin decreases. Best reservoir properties are found within the early stage of the turbidite parasequence, which is around 100 ft thick.The late stage turbidites are isolated in the shaly background sedimentation. Mapping of sandstone content within the early stage turbidites of parasequence 5 was carried out using a GR cut off of 90 API. It shows a lobe like distribution with increasing sandstone amount from the top of the hanging wall towards the deep graben area, which is next to the basin margin (Fig. 3). Seismic attribute analysis of acoustic impedance within a time window across the early stage sandstone interval shows high acoustic impedance contrasts within the deep graben areas, which might indicate an increase of sandstone thickness.The Triassic sandstone drilled in the As Sarah field belong to a distal turbidite fan facies, which was deposited around 5 km SW from the basin margin.

Most of the wells in the As Sarah field found the Triassic sandstones oil bearing, with porosities of around 10% but poor permeabilities in the range of 1 - 10 MD. One well was tested and proved oil flowing.

Reservoir quality is predicted to improve with increasing grain sizes towards the deep graben areas, proximal to the basin margin (Supra Fan).

Conclusions

The Maragh Trough is a Triassic rift basin in the Eastern Sirt Basin related to the break up of Pangaea. Syn-rift sediments comprise source rocks and reservoir rocks in the form of deep water turbidites.The best reservoir facies is to be found in the deep graben areas close to the basin margins.The turbidite sandstones are embedded in a shaly background sediments, which act as source and seal. It is a stratigraphic trap type.These reservoirs are a new type of exploration target in Libya to be found in the deep basins off structural highs.

Fig. 1. Reflector geometry of the Triassic half-graben fill--NE of the As Sarah oilfield.

Fig. 2. B26-96 type well - Triassic syn-rift lithology.

Fig. 3. Percentage of sandstone (GR cutt off < 90 API) within the early stage interval (~ 100 ft) of Parasequence 5.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England