Publications

The reservoir quality of the Lower Miocene sandstones in wells on the VICO-operated Sanga- Sanga PSC have been evaluated. The controls on porosity, permeability and cement distribution are described and a diagenetic model proposed. Data from Middle Miocene sandstones have been used to substantiate the model.The Lower Miocene sediments have been penetrated in wells in the west of the block (Foldbelt area) and in the Wain sub-basin area (Figure 1). Sandstones and shales are the dominant lithologies with subordinate coals and limestones.Cross plots of porosity versus sandstone netlgross ratio show that in the Wain sub-basin, the Lower Miocene sandstones are of better reservoir quality than those to the north and west in the Foldbelt area (Figure 2). For a porosity cut-off of l0%, there is a sandstone net/gross ratio of 55 +- 5% in the Wain sub-basin but only 7 +- 5% elsewhere. These ratios appear to be independent of facies and original composition, although ductile volcanogenic clasts do reduce porosity upon burial.Burial depth, temperature and the related maturation of carbonaceous material and, in particular, pressure are the major controls on the diagenesis of the sandstones. Upon increasing burial. the sediments pass from a normal, hydrostatically pressured zone through a transition or ",Gasex", zone, between 8000' - 12000' maximum burial, into the ",Hard", overpressured zone. Disequilibrium compaction is considered to be the main coiitrol on the distribution of these zones.The ",Gasex", zone has a temperature range of 100 - 120°C and corresponds to the Lone of oil to gas conversion at Ro = 0.6 and the expulsion of hydrocarbons from the coal source rocks In the example from a SaLakanan well. which has undergone some 7000' of uplift, above the ",Gasex", zone, porosity occlusion is by early carbonate cement and, where present, the compaction of volcanogenic grains (Figure 3) Siderite cement IS present throughout but shows no clear lariation in this zone Upon burial to within the ",Gasex", zone, any carly-formed carbonate cements arc dissolved by the carboxylic acids generated during the maturation of the coal source rocks (Figure 4).In the ",Hard", overpressure zone. the precipitation of carbonate cements is the main cause of further porosity destruction with the acme of this cement just below the top of the zone (Figure 4). Siderite and silica cements are also present.In the wells in the Foldbelt of the Sanga-Sanga PSC the carbonate cements are common in those sandstones that were buried to in excess of 11500' and. hence, were in the ",Hard", overpressure zone This is the main reason that these sandstones have poorer reservoir quality than those in the Wain subbasin.