Conditions of Forming and Geological Specifics of Non-structural Traps in the Ordovician Deposits of the Illizi Basin (The Algerian Sahara)

摘要

The exploratory works in oil-and-gas bearing basins of the Northern Africa were commenced as early as in the middle of the 20th century. One of the most important and probably the most examined oil-and-gas bearing basins in Algeria is the Illizi basin. In 50 years of the prospecting exploratory works in the territory of the Illizi basin with the area of 108 424 km~2 more than 700 exploration and deliniations wells were drilled leading to discovery of more than 100 oil (57) and gas (49) fields with the total geological resources of more than 1.7x109 m~3 of oil and more than 582x109 m~3 of gas. The largest deposits discovered here are Tin Fouye and the Ohanet group of deposits. According to estimates of specialists (B. Gougam; M.EH Boumghar. Sonatrach-Exploration / Boumerdes-Algeria), the oil-and-gas bearing potential of the Illizi basin may reach up to 40 million tons of oil and up to 8500 billion m3 of gas, where up to 70% of gas and 20% of oil trapped in the Cambrian-Ordovician reservoirs. Even agreeing with the fact that these estimates are substantially overstated, this territory still remains to be attractive. This can be judged based on the dense distribution of the discovered reserves of hydrocarbons compared with the degree of the knowledge. According to investigations of Macgregor and Traut (D. S. Macgregor. Giant petroleum fields of Algeria, 1998, M. Traut at al., Exploratory history of the Palaeozoic petroleum systems, 1998), the main quantity of the deposits discovered in the Illizi basin are referred to structural (simple anticlines, tectonically restricted anticlines) and / or, to lesser extent, to combined traps. Complex depositional stratigraphical traps, and their alike, so called subtle traps, which as a rule are confined to the Cambrian-Ordovician deposits, are an object of close attention of the companies, which carry out exploratory works in this territory. One of the main problems during prospecting works related to complex Cambrian-Ordovician traps is sharp lithic variability of the cross section against the background of an intricately built structural stratigraphical complex on the whole. This issue was shown in many pappers based both on the regional correlation of key wells and on interconnecting the data describing the outcrops with data of the wells. The basis of this article is the materials obtained by Rosneft-Stroytransgas Limited (RN-STG ltd) during prospecting exploratory works in the central part of the basin and analysed jointly with published materials and data of special investigations. As a result, stratification of the Cambrian-Ordovician deposits of the central part of the basin in its northern has been clarified in its northern closure, and the rocks were characterized and the propagation of their collect collecting properties was predicted. The below presented are the main obtained results, and also some conclusions with respect to the principle moments related to distinguishing and mapping of non-structural traps of such type.