Aïcha Hamel¹, Nacéra Maache¹, Madani Benhamouda²

(1) Sonatrach, Algiers, Algeria
(2) Sonatrach, Boumerdes, Algeria

ABSTRACT: Sequence Stratigraphy, Reservoir Architecture of Rhourd Nouss Triassic Sandstones and Zarzaïtine Outcrop Analog (Algerian Sahara)

Rhourd Nouss gas condensate field is producing from Triassic sandstones. An excellent core coverage enables detailed study, which shows a cyclic depositional pattern, with mainly fluvial sandstones.

The wells spacing exceeding two kilometers did not allow for enough resolution to built a 3D geological model to be used for reservoir simulation. A detailed field study at a reservoir scale has been undertaken on outcrops in the Zarzaïtine region, 300 Km south-east of Rhourd Nouss field.

Bed scale reservoir architectures were quantified with photo-mosaics and by correlation of measured sections. Three depositional sequences were identified; only the upper one corresponds to Rhourd Nouss reservoirs. As sediments came from the south-west, as proven by NE paleocurrent directions, the fluvial series thicken northward.

The outcrop fluvial channels correspond to low-sinuosity river systems. The channel fill often consists of large coarse-grained fluvial bars. Parallel horizontal laminated sandstones are interpreted as overflow deposits, reworked periodically by aeolian processes. The reservoir architecture of the fluvial sand sheet is complex, because of the amalgamation of tens of single channel sequences in a reservoir unit. Rapid thickness variations are linked to interfingering of fluvial channel sandstones in the floodplain mudstones.

Although the setting is tectonically different, Rhourd Nouss reservoirs geometry seems similar, even they are mainly made of coarse grained sandstones, deposited in distal alluvial fan or proximal braided streams. They evolved distally to poorly channelized fluvial sand-sheets interbedded with anhydrite shale beds. These sand-sheets are interpreted as braided fan-deltas prograding northward into sabkha deposits.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado