Serraria Formation

The aims this study are to elucidate the factors controlling the distribution pattern, mineralogy and geochemical composition of calcite and dolomite cements, and of grain dissolution and kaolin-ization in the fluvial sandstones of the Serraria Formation, northeastern Brazil (Fig. 1). This unit provides an excellent opportunity to examine the influence of variations in the palaeogeographical setting, palaeoclimate and burial history on diagenetic processes. 

Fig. 3. Palaeogeographical reconstructions of Serraria Formation. Early Cretaceous pre-rift sedimentation. (A) At the time of maximum extension of the lacustrine system and beginning of the fluvial sedimentation of the Serraria sandstones (B) at the time of maximum expansion of the Serraria fluvial system. Modified from Garcia (1992).

Fig. 4. Stratigraphical column of Sergipe-Alagoas Basin. Serraria Formation (arrow) is part of the continental pre-rift sequence.

The Serraria Formation consists of six lithofacies intervals (Garcia, 1992). The sedimentary structures, facies interpretation and thicknesses of each interval are shown in Fig. 5. The lower interval 1 and upper interval 5 are composed of interbedded... 

Fig. 6. QFL detrital composition of the sandstones of the diagenetic domains of the Serraria Formation plotted on a Folk (1968) classification diagram. The proximal domain is closer to the original detrital composition.

The framework grain composition and palaeo-current analyses indicate that the source rocks for the Serraria Formation were mainly granitic-gneissic rocks, schists and quartzites of the Pre-Cambrian Complex bordering the northern sector of the Afro-Brazilian depression (Garcia, 1992) (Fig. 3). Other less important source rocks include a volcanic suite. 

Inthe proximal Furado area, subsidence was rapid and continuous from the beginning of the syn-rift burial phase (i.e. 140-135 Ma) to 125 Ma, bringing the Serraria Formation to approximately the present maximum depth of 2500-3000 m ( 100-120°C) (Fig. 20) in the deeper faulted blocks. 

At present the Serraria Formation is exposed in some parts of the middle domain (Japoata-Penedo area). In outcrops to shallow burial sectors of this domain the present burial depth is 50-900 m (Fig. 21). Depths of up to 2000 m are estimated for the syn-rift subsidence phase (wbO-SO C) in this domain. After the Lower Cretaceous uplift, depths of a 3500 m were attained in deeper blocks ( 100°C). 

Rapid burial during the rift phase brought the Serraria Formation down to a maximum depth of i 1500-2500 m. The diagenetic evolution of the sandstones during this burial phase has varied between the different domains, mainly due to vari-... 

Fig. 21. Palaeogeological map of the southern Sergipe-Alagoas Basin at aj74 Ma, showing the areas of exposure of the Serraria Formation at the maximum development of the post-rift, Pre-Muribeca unconformity (Ojeda, 1982). The average remaining feldspar content after meteoric flushing in the studied wells increases with distance from the unconformity, as illustrated by section A-B.

The patterns of diagenetic evolution recognized in this study allow discussion of the conditions for optimum porosity preservation and/or enhancement in the Serraria reservoirs. The best reservoirs of the unit occur in the Caioba area of the distal domain, where porosity was enhanced by dissolution of detrital feldspars and dolomite cement during telogenetic influx of meteoric waters. Similar conditions are expected for other structural blocks of the basin affected by post-rift uplift and erosion, or blocks bounded by major fault systems in which the Serraria Formation was relatively close to the... 

The best reservoir quality potential expected for the Serraria Formation is in structural blocks in the distal and middle domains affected by porosity enhancement through extensive feldspar and carbonate cement dissolution in connection with the post-rift exposure and telogenetic infl ux of meteoric waters. 

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