Fractured Granite Basement Reservoir Discoveries in the Bongor Basin of Chad

Abstract

The Bongor basin is located in southwest of Chad and a Mesozoic-Cenozoic rift basin developed along the Central African Shear zone associated with the opening of the Atlantic Ocean during the Early Cretaceous. It covers an area of approximately 18,000 km² (280km * 40–80km). This basin was initiated by a NE-SW trending extension and developed numerous of NW-SE trending depressions during the Early Cretaceous. The tectonic inversion in Santonian time resulted in the wide-spread basin uplift and strata deformation. The sedimentary rocks from the Lower Cretaceous to Cenozoic uncomfortably overlie the Precambrian or metamorphosed basement with a thickness over 10 km. The basin is composed of four major structural units, i.e., the northern slope, central depression, southern uplift and southern depression. Fractured basement is faulted hills or rotated horst blocks; the matrix between hydrocarbon-filled fractures is composed of granites and granitoidites. Oil potentiality of naturally fractured basement reservoirs in Bongor basin is manifested by the discovery of Lanea-1 well in the early 2013. Subsequently eight “buried hill” belts have been recognized in the north of slope, aligned along the sub-parallel NW-SE trending; and five of them have been targeted as hydrocarbon-bearing reservoirs by drilling. The source rocks are in the Lower Cretaceous overlying or laterally contacted with fractured basement; the Early Cretaceous lacustrine muds constitute regional seals. Reservoir quality is controlled by tectonics, weathering and hydrothermal alteration in fractured basement play in Bongor basin and advance seismic technologies play a key role in integrated characterization of highly heterogeneous reservoirs. This play can be divided into four depth-related seismic zones of the weathered, fractured, leached, and weakly altered and fractured. Geometric attributes such as coherence and curvature presented two dominant strike orientations of NE-SW and NW-SE of faults and tectonic fractures formed by the extension and inversion during the Cretaceous time which resulted in the development of the secondary porosity within the reservoir. Effective porosity ranges 4 - 12%, occasionally up to 30%. The high quality of seismic imaging showed the top surfaces of the buried hill structures are usually at 500 to 1500 mss with up to 1500 m oil column. The crude oil is characterized by light oil and the flow rates per well is in 1500 BOPD.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015