PALAEOZOIC SANDSTONE RESERVOIRS OF THEHAMADA BASIN, NW LIBYA:

EFFECTS OF SYNSEDIMENTARY PROCESSES ONPOROSITY

S. H. A. Shah*+, A. Mansouri* and M. El Ghoul*

The Hamada Basin of NW Libya is an intracratonic basin in which a thick sequence of clasticrocks was deposited during the Palaeozoic. These sediments were derived from a PreCambrianbasement of felsic igneous and metamorphic rocks, and include both sands and clays; almost all theclays in formations of Palaeozoic age were produced from the alteration of feldspar. Structures weredeveloped as these sediments were draped over the surface of the basement.

Reservoirs in the Lower Palaeozoic (Lower Devonian, Lower Silurian and Cambro-OrdovicianSandstones) appear to have developed as a result of synsedimentary processes, such as winnowingand washing-out of clayey material from the sand by percolating waters, and sliding and slumping onthe steep depositional surface during or soon after deposition. Oil traps are independent of structuralelevation, but appear to have developed on those surfaces which were sufficiently steep to haveinitiated the above synsedimentary processes.

This theory has been successfully tested in many places in the basin, and may provide a lead forexploration in other basins in Libya that have similar geological settings.

INTRODUCTION

The Hamada (Ghadames) Basin is located in NW Libya, and extends into southern Tunisia andeastern Algeria (Fig. 1).

Recent exploration studies have resulted in the development of a new concept for oil explorationin this and other similar basins in Libya. This is applicable to Palaeozoic sandstone reservoirs whichhave developed effective porosity and/or permeability as a result of various synsedimentaryprocesses. The gradient of slope at which these processes were effective can be determined fromseismic maps.

The slope concept was successfully tested in the Hamada Basin as an exploration tool for oil andgas in different sandstone pay-zones.

Journal of Petroleum Geology, vol. 16(3), July 1993, pp. 345-352 345

* Exploration Division, Arabian Gulf Oil Co., Benghazi, Libya.+ Current address: Dept. of Geology, University of Karachi, Karachi, Pakistan.

General Geology

The Hamada Basin is an intracratonic basin of Palaeozoic-Mesozoic age, in which a thicksequence of clastic rocks was deposited during the Palaeozoic. The basin is bounded structurally andtopographically by the Nefusa Uplift to the north, the Tripoli-Assuda Arch in the east the GargafUplift in the south, and the northern extension of the Thampoka Arch in the west (Fig. 1).

The basin is characterized by a series of highs trending NE-SW that reflect the PreCambriantopography. These structures appear to have formed as a result of draping and compaction ofsediments as they were deposited over the PreCambrian basement. The basin itself was subject toseveral epeirogenic movements (Bishop, 1978), that resulted in a regional Hercynian angularunconformity, as well as several disconformities (Fig. 3 page 348).

The sedimentary sequence in the central part of the basin, near the frontier with Tunisia, is morethan 19,000 ft thick (Mikhbel 1977). The Palaeozoic section alone comprises about 14,000 ft ofinterbedded sandstones and shales. The sandstone occurs in sheet-like bodies that reflect differentdepositional environments. Carbonate rocks within the Palaeozoic section do not exceed 300 ft inthickness. A generalized stratigraphic section for the basin is shown in Fig. 3.

Clastic rocks in most localities in the basin contain variable amounts of clay materials. The sourceof these rocks is believed to be the PreCambrian igneous and metamorphic basement which surroundsthe basin as structural highs, and clays probably developed as weathering products from the feldsparsthat make up the bulk of the basement rocks.

OIL OCCURRENCE IN PALAEOZOIC SANDSTONES

Oil and gas have been produced from seven Palaeozoic sandstone pay-zones (Fig. 2). The sourcerocks are mainly Silurian shales (Byramjee and Vasse, 1969), supplemented by Devonian andCambro-Ordovician shales (Beicip, 1972). Reservoirs range in age from Cambro-Ordovician touppermost Devonian or lowermost Carboniferous.

The net pay of the producing sandstones is limited in area, ranging in thickness from 20 ft to 50 ft,and appears to be confined to zones of reasonably high slope-gradient. The porosity of the sandstonereservoirs differs not only from one horizon to another, but also from one locality to another within asingle horizon. The average porosity for most of the productive sandstones, however, ranges from13% to 18%.

The productivity of Cambro-Ordovician, Lower Silurian, and Lower Devonian sandstones iscontrolled by their stratigraphic-structural setting. The present study shows that the accumulation andentrapment of oil is confined to structures whose crests are narrow and flanks are steep(Fig. 2), and that neither structural elevation nor amplitude plays a significant role in oilaccumulation and/or entrapment. Moreover, the position of the pay-zone may be completelyindependent of closure (Fig. 5).

Lower Palaeozoic sandstones are characterized by higher porosities in the structurally-higher partsof the dipping sand-bodies. Lower porosities are expected in the lower parts of the sand-bodies, aswell as the flat portions of uplifted structures (Figs. 4 and 5).

Based on this study, it is deduced that the high-porosity sandstones were subjected to sedimentaryprocesses that enhanced their porosity, such as the flushing-out of clayey material prior toconsolidation and burial of the sands. As the clays are more abundant in the flat or gently-slopingareas, a direct relationship between the gradient of the depositional surface and the synsedimentaryprocesses, which enhance the sandstone porosity, can be established.

Palaeozoic sandstone reservoirs, Hamada Basin, Libya346

SYNSEDIMENTARY PROCESSES

Processes such as winnowing, washing, reworking, sorting etc operating at high hydraulic energy

levels, appear to have acted on the sand before its consolidation and burial. These processes, which

may have been aided by gravity would not have been effective unless the depositional surface

attained a threshold gradient; the more pronounced the gradient and the longer the time involved the

greater the effect of these processes would be and, in turn, the better the resulting reservoir quality.

Although a detailed investigation of these processes is yet to be carried-out, the pattern by which

they appear to have enhanced the porosity and permeability of the Lower Palaeozoic sandstones is as

follows:

1. The fine-grained matrix of the semi-consolidated sandstone was flushed-out through

intergranular spaces down-slope. This flushing was caused by percolating waters derived from

retreating waves and tides (Fig 6).

2. The fine-grained matrix was flushed-out through conduits created by gravity gliding, sliding

and slumping of the particles in the unconsolidated sand, which may have increased the volume of the

sand-body by creating new voids or expanding existing ones.

3. The fine material would have been flushed-out basinwards through numerous planar micro-

fractures that developed due to sliding and slumping of the sand-body in response to constant

agitation by wave-action and tides, aided by gravity.

4. The fine materials would have been removed as the angle of repose of the sand-bodies was

disturbed by intermittent shocks from earth quakes or storm waves. Such shocks would facilitate

sliding and shaking of the sand-body as well as reworking the sand after each disturbance.

5. The clay particles remaining in the intergranular space in the sand would have been orientated

with their longer axes in the direction of flow (Fig. 7) thus enhancing permeability by enlarging the

pore-throats. These processes would locally provide a seal by flushing the swollen clay to the

structurally lower parts of the sand-body in a manner depicted as a structural nose (Fig 8).

Palaeozoic sandstone reservoirs, Hamada Basin, Libya350

CONCLUSIONSOil from the Palaeozoic section in the Hamada Basin is produced from at least seven sandstone

pay-zones, sourced mainly from Silurian shales, but supplemented by Cambro-Ordovician andDevonian shales. The productivity of some of the sandstones is independent of structural elevation,closure or amplitude, but is essentially related to the gradient of the area; the more pronounced thegradient, the greater the porosity and permeability.

Enhancement of porosity and permeability of the Palaeozoic sandstone reservoirs is related tosynsedimentary processes that acted upon the sediments prior to consolidation and burial. Theseprocesses had the greatest effect where the slopes were steep, and acted principally by flushing theclay materials down-slope. For such processes to be active, a threshold gradient was required, and thisvaried from one area to another.

ACKNOWLEDGEMENTSThe Authors wish to acknowledge the National Oil Corporation and the Arabian Gulf Oil Co. for

granting permission to publish this paper, which is based on a contribution originally compiled for the27th International Geological Congress. Thanks and appreciation are due to Mr Faraj Said, Chairmanof the Board, and Mr Ahmad Asbali, General Exploration Manager, for their encouragement andsupport. Critical review by Dr. M. Ala (Imperial College) is acknowledged. D. Minter helped withdrafting of Figures.

S. H. A. Shah et al. 351

REFERENCESBEICIP, 1972. Western Libya Exploration Study, Unpub. Rep., NOC.BISHOP, W.F., 1976. Geology of Tunisia and adjacent parts of Algeria and Libya. AAPG Bull., 59/3,

423-450.BYRAMJEE, R. and VASSE, L., 1969. Geochemical interpretation of Libyan and North-Sahara

crude oil analyses. I n : Advances in organic geochemistry, 1968. Pergamon Press, Oxford, pp.319-330.

MIKHBEL, S.R., 1977. Basement configuration and structure of Western Libya. Libyan Journ. Sci.,7, 19-34.

Palaeozoic sandstone reservoirs, Hamada Basin, Libya352