This paper examines the diagenetic history of dual (i.e. matrix and fracture) porosity reservoir lithologies in Cretaceous to Eocene carbonate turbidites of the Ionian fold and thrust belt, close to the oil-producing centre of Fier-Ballsh (central Albania). The first major diagenetic event controlling reservoir quality was early cementation by isopachous and syntaxial low-Mg calcite. These cements formed primarily around crinoid and rudist fragments, which acted as nucleation sites. In sediments in which these bioclasts are the major rock constituent, this cement can make up 30% of the rock volume, resulting in low effective porosity. In strata in which these bioclasts are mixed with reworked micrite, isopachous/syntaxial cements stabilized the framework, and matrix porosity is around 15%. The volumetric importance of these cements, their optical and luminescence character (distribution and dull orange luminescence) and stable isotopic signal (d 18 O and d 13 C averaging respectively; )0AE5& VPDB and +2& VPDB) all support a marine phreatic origin. Within these turbidites and debris flows, several generations of fractures alternated with episodes of cementation. A detailed reconstruction of this history was based on cross-cutting relationships of fractures and compactional and layer-parallel shortening (LPS) stylolites. The prefolding calcite veins possess orange cathodoluminescence similar to that of the host rock. Their stable isotope signatures (d 18 O of )3AE86 to )0AE85& VPDB and d 13 C of -0AE14 to + 2AE98& VPDB) support a closed diagenetic rock-buffered system. A similar closed system accounts for the selectively reopened and subsequently calcite-cemented LPS stylolites (d 18 O of )1AE81 to )1AE14& VPDB and d 13 C of +1AE52 to +2AE56& VPDB). Within the prefolding veins, brecciated host rock fragments and complex textures such as crack and seal features resulted from hydraulic fracturing. They reflect expulsion of overpressured fluids within the footwall of the frontal thrusts. After folding and thrust sheet emplacement, some calcite veins are still rock buffered (d 18 O of )0AE96 to +0AE2& VPDB and d 13 C of +0AE79 to +1AE37& VPDB), whereas others reflect external (i.e. extraformational) and thus large-scale fluid fluxes. Some of these veins are linked to basement-derived fluid circulation or originated from fluid flow along evaporitic décollement horizons (d 18 O around +3AE0& VPDB and d 13 C around +1AE5& VPDB). Others are related to the maturation of hydrocarbons in the system (d 18 O around )7AE1& VPDB and d 13 C around +9AE3& VPDB). An open joint system reflecting an extensional stress regime developed during or after the final folding stage. This joint system enhanced vertical connectivity. This open joint network can be explained by the high palaeotopographical position and the folding of the reservoir analogue within the deformational front. The Sedimentology (2002) 49, 697-718 Ó 2002 International Association of Sedimentologists 697joint system is pre-Burdigalian in age based upon a dated karstifie...

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Zebra dolomites, characterized by a repetition of dark grey (a) and light (b) coloured dolomite sheets building up abbabba‐sequences, occur in Dinantian strata from deep boreholes (> 2000 m) south of the Brabant‐Wales Massif in Belgium. These zebra dolomite sequences are several tens of metres thick. The dark grey dolomite sheets (a) consist of non‐planar crystals, 80–150 μm in diameter. These crystals display a mottled red–orange luminescence and are interpreted to be replacive in origin. The white dolomite sheets (b) consist of coarse crystalline nonplanar b1 dolomite, which evolves outwards into transparent saddle shaped b2 dolomite. The b1 dolomites possess a mottled red–orange luminescence similar to the a dolomites, while the saddle shaped b2 rims display red to dark brown luminescent‐zones. The b1 dolomites are possibly partly replacive and partly cavity filling. Their b2 rims display criteria typical for a cement origin. Locally, cavities exist between two succeeding white dolomite sheets. These cavities make up ≈5% of the zebra rocks and are locally filled by saddle shaped ankerite and/or xenomorphic ferroan calcite. Geochemical and fluid inclusion data (Th ≈ 120 °C) indicate a burial diagenetic origin for these zebra dolomites. The a and b1 dolomites are characterized by similar geochemical compositions and fluid inclusion data pointing toward a related origin. To explain the development of the zebra textures, suprahydrostatic pressures in conjunction with late Variscan tectonic compression are invoked. A model involving dolomitizing fluids expelled during the Variscan orogeny is proposed. An overpressured system is also invoked to explain the important porosity development, the creation of centimetre‐scale subvertical displacements of the zebra pattern and the microfractures affecting the b1–b2 dolomite crystals.

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