Critical factors in the exploration of an Atlas intramontane basin; the Western Hodna Basin of northern Algeria

Abstract

Abstract The acquisition and evaluation of a substantial integrated surface outcrop and well database over the western Hodna Basin allows the identification of three critical factors (reservoir quality, source maturity and timing and hydrodynamic regime) to which well failures to date can largely be attributed. Reservoir quality is variable in Cretaceous sandstones, because of a high degree of quartz cementation, but improves to the south. Reservoir quality in Eocene limestones is related to the development of vuggy porosity and can now be mapped out from well control. Potential Cretaceous source rocks lie at moderate to high degrees of maturity, with explusion probably preceding the Miocene phases of structure development. Tertiary palaeostress reconstructions from field studies indicate four main structural phases: (1) Late Cretaceous N-S to NW-SE oriented compression; (2) Oligo-Miocene distension; (3) N-S oriented Miocene (pre-Langhian) pull-apart; (4) Langhian compression related to the emplacement of the Tellian nappes. Seismic analysis illustrates some classic late-stage inversion structures. Organic matter seen in outcrops and cores within the Thanetian-Ypresian and lower Lutetian sections is composed mainly of type II kerogen and varies from undermature to marginally mature, leading to the development of shows in many wells that helped in the past to stimulate continued exploration. Study of the hydrology of the formation waters in drilled wells allows large areas of likely reservoir flushing to be delineated. Study of the three critical factors allows a high grading in terms of remaining exploration potential. Based on the interpretations derived from structural, palaeogeography, diagenetic and hydrodynamic studies, the basin can be divided into three areas, which are ranked on the basis of their prospectivity. Each of the factors also needs to be critically assessed in analogous regions of the Atlas, in which such good geological control may not exist.