Palynostratigraphy and palynofacies of subsurface devonian (middle-upper) strata of the Ne Al-Rar gas and oil field, N.W. Libya

Abstract

Lithological and palynological analyses of the Upper and Middle Devonian strata of the NE Al-Rar Gas & Oil Field (NW Libya) have been carried out, on ditch cutting and conventional core samples from five boreholes (Al-A5).

186 samples have been prepared for microscopic investigation. From these samples, 112 palynomorph species with stratigraphic value have been recorded, six of which are new. Seven palynological zones have been identified, ranging in age from early Givetian to latest Famennian. Correlation of these palynological zones within the Ghadames Basin and with other regions has been attempted.

These palynological zones indicate that sediments from the Tahara Formation are mainly late & latest Famennian in age, sediments from the Awaynet Wanin C Formation are late-mid Fammenian age, sediments from the Cues Limestone Horizon (the highly radioactive shale unit) are probably latest Frasnian in age, the Awaynet Wanin B Formation is late Early-Mid Givetian in age and sediments from the F3-Sand (the main reservoir) & the uppermost part of the F4-Sand Members (Awaynet Wanin A Formation) are early Givetian in age.

The absence of the most characteristic Frasnian & late Givetian spores, which have been found in other parts of Libya and neighbouring areas (Algeria & Tunisia), lead to the conclusion that the sediments from the Awaynet Wanin B Formation are unconformably overlain by the highly radioactive shale (Cues Limestone Horizon).

Eighteen sedimentary facies & five palynofacies have been identified from the F3-Sand (the reservoir), the adjacent strata and the highly radioactive unit (Cues Limestone Horizon) enabled the development of paleoenvironmental model for these sequences as follows:

I- Both the upper part of the F4-Sand and the A.W.B. formation were deposited in relatively deep (Offshore), dysaerobic - anaerobic, occasionally interrupted by more oxic marine conditions.

II- The F3-Sand (reservoir) was deposited in shallower (shore face), and highly oxic, marine conditions.

III- The highly radioactive unit (Cues Limestone Horizon) was deposited in deeper, anaerobic and stagnant marine conditions.

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