| Popis: |
The Golfo San Jorge area in central Argentina represents an intraplate basin consisting of several sedimentary formations, the oldest being of Jurassic age. The Cretaceous fluvio-lacustrine Bajo Barreal Formation, known for its large hydrocarbon accumulations, makes the upper portion of the basin fill and consists of an upper and lower stratigraphical Members laying above a well known regional tuffaceous marker. The Lower Member consists of floodplain mudstones with isolated channels and has an upward increase of sandstone content ; the Upper Member is composed by grey and purple mudstones with thicker channel sand bodies. Both stratigraphical units are characterised by the presence of acid volcaniclastics and tuffaceous material mixed with siliciclastic sediments. Previous studies on the Bajo Barreal Formation suggest an active sedimentary input from contemporaneous volcanoclastic material derived from both direct fall-outs or rain-off processes. This material can make up to 15% of host rocks forming a so-called pseudomatrix produced by the disintegration of tuff and d pyroclastic material. In this contribution we present the preliminary results on the reservoir petrography and mineralogy focusing on the characterisation of its volcanic component and related neoformed mineralization especially considering the clay and zeolite pore infill. The methodological approach include QEMSCAN®, SEM-EDS, and XRD studies on bulk samples and their clay fraction. Our data from 5 wells show the reservoir rocks to be mainly formed by mudstones to fine- to medium grained sandstones (Figure 1.). Their mineralogy comprises quartz and feldspars, whilst volcanic component occasionally makes up to 50 % of the sample, resulting in form of very fine- grained tuff or volcanoclastics particles up to 2 mm in size. Their composition is presumably acidic, as indicated by the variety of albite, andesine, and quartz of poor grain homogeneity and unusual shapes detected with the QEMSCAN®. Corroborating the presence of volcanic material, XRD whole-rock data indicate high contents of amorphous matter (i.e. 25-35 °2Θ bulging) and much lower contents of crystalline feldspar than initially suggested by QEMSCAN® data. Alkali-rich volcanic component is readily altered to the range of secondary assemblages consisting primarily of clay minerals-like mixed-layered illite-smectite (I- S), chlorite, and kaolinite as well as zeolites stemming from the group of true zeolites (heulandite, clinoptilolite, or laumontite). Mixed-layered I-S is rich in dioctahedral smectite component (60-90% Sm) and it is mostly formed after tuffaceous hypohyaline matrix. Chlorite, on the other hand, stands out after volcanoclastics thus indicating a possible difference in volcanism. Kaolinite is replacing feldspars. The micro-textural context of zeolites is at the moment unknown due to their “exotic” chemistry that hampered their proper QEMSCAN® classification. Clay content can make up to 15% of analysed rocks, whereas XRD Rietveld analyses quantify zeolites in amounts not exceeding 5%. Vertical current depth distribution of clays and zeolites is particularly interesting as I-S seems not to be present at depths shallower than 800 m, and deeper than 1400 m. Zeolites are reported in higher amounts at shallower depths than 800 m whilst at 1400 m and deeper chlorite is a dominant clay phase. For kaolinite no distribution pattern could have been elucidated. While a clear diagenetic sequence is still to be established, a clear trend in porosity values has been identified by a negative correlation with I-S abundances being significantly reduced at horizons with maximal content of this interlayered smectite-rich mineral. The identification and quantification of zeolites rich intervals has also an impact on porosity calculation from density-based wire- line log and allow a re-assessment of the stratigraphic distribution of reservoir properties. Ultimately, the results of this study will be used to define better the exploitation practices (e.g. EOR techniques) and thus increase reservoir productivity. |
