Petrography and origin of dedolomites of the Ordovician Majiagou formation in the southeastern Ordos Basin, China: implications for reservoir quality

Dedolomites, the replacement of dolomite by calcite, are widely distributed in the Ordovician Majiagou Formation of the southeastern Ordos Basin, China, which critically affects reservoir quality throughout the region. Two types of dedolomites were recognized in the upper 100 m of the Majiagou Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic and geochemical data reveal that the investigated dedolomites are often associated with clay minerals and dissolution-collapse breccia, indicating their close relationship with erosional unconformities. The relative negative δ18O and δ13C values and relative high Fe, Mn contents of dedolomite with respect to corresponding dolomite are interpreted as being result from meteoric phreatic water involvement. The dedolomitization process related to subaerial exposure is conventionally interpreted to be formed by dolomite dissolution and calcite precipitation. The rhombic shape crystals in the brecciated dedolomite was formed by an Mg2+-loss process under evaporite solution conditions, which retain the rhombic shape of the dolomite. The crystalline dedolomites, however, were formed by dissolution/precipitation and recrystallization process to form medium to coarse calcite mosaic with very low intercrystalline porosity. The resulting fabrics are mainly controlled by the paleo-topography. The precursor dolomite of the crystalline dedolomite is primarily recrystallized mudstone, which is more soluble and favorable for calcite precipitation. The crystalline dedolomite mostly occurs in the low paleotopographic locations, where the karst-saturated water with respect to CaCO3 is more concentrated, occluding the remaining porosity when excess calcium is supplied. It’s, therefore, recommended to implement drilling in tectonic highland and avoid low paleotopographic locations.

Petrological and Biostratigraphic Characteristics of Pre-Cenozoic Carbonate Rocks in the Northern Song Hong Basin, Vietnam

Pre-Cenozoic carbonate rocks in the northern Song Hong basin, Vietnam that are being considered and studied by oil companies in exploration and exploitation. The hydrocarbon accumulations in these rocks have been discovered and have significantly commercial reserves, in which the porosity plays an important role in estimating the capacity of hydrocarbon. The carbonate rocks are composed mainly of crystalline limestone, packstone, wackestone and mudstone, which have been experienced dolomitization, compaction and dissolution. The main carbonate pore systems include fracture, vuggy and intercrystalline porosity. The predominance of larger benthic foraminiferal assemblages indicates that the carbonate sediments were formed during the late Paleozoic (Carboniferous-Permian) and were deposited in shallow marine environment. Furthermore, the obtained petrological and biostratigraphic characteristics are well-correlated with the carbonate formations exposed in adjacent Cat Ba island area. The results of this study are either used in petroleum exploration or used in a local stratigraphic correlation in northern Vietnam.

Influence of use of sinteringtechnology on the structure of materials in the silicon carbide —bor system.

In the present work, we examined the effect of the use of pre-sintering technology on the structure of ceramic materials in silicon carbide —boron system. The influence of pre-sintering technology use on the formation of the structure of ceramic materials with the content of boron additive (SiC–xB, where x= 5, 10 wt. %) is determined. Hardness was measured for the obtained materials and scanning electron microscopy was performed. Materials obtained by classic technology had a monolithic structure and porosity characteristic of the solid-state sintering mechanism. The materials obtained by the pre-sintering technology had an equal crystalline structure with low intercrystalline porosity and an almost complete absence of regions with high coherence of boundaries. The highest hardness (27,2 GPa) was possessed by a material containing 10 wt. % boron and obtained by classic technology.

An integrated approach to quantify geologic controls on carbonate pore types and permeability, Puguang gas field, China

Accurate estimation of permeability for reservoir simulation and production is challenging in carbonate rocks due to the diversified pore structures resulting from deposition and diagenetic modification. A significant amount of residue gas is expected in the Puguang field, China. We use a shear-frame flexibility factor [Formula: see text] from a rock-physics model as an index to quantify its spatial variation of pore structure and constrain the estimation of permeability in this field. The pore-structure index [Formula: see text] is established and used to classify various permeability-porosity trends at well locations where core and log data are available. It is found that when [Formula: see text], the pore type is dominated by intercrystalline pores with large pore-throat sizes and high connectivity, the permeability-porosity relation is [Formula: see text]; when [Formula: see text], the pore type consists of isolated moldic pores, the permeability-porosity relation is [Formula: see text], where [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] are constants, which are 80, 18, 3, 0.004, 5.5, and 1.6, respectively, for the studied gas reservoir. Rock-physics-based seismic inversion is then applied to quantify the spatial variation of pore type and permeability. The inversion results indicate that regional stratigraphy has a paramount control on the distribution of pore type and permeability. Moldic pores ([Formula: see text]) are widely distributed near the unconformities, whereas the large intercrystalline porosity and microporosity are distributed below and above the unconformities due to the sea-level regression and transgression, respectively. It is concluded that production problems may occur in porous and permeable intercrystalline-porosity zones and the exploration of residual gas remains in the high porosity yet less permeable moldic-porosity zones.

Controls on diagenesis of the Triassic Kurrachine Dolomite, Syria

ABSTRACT The evaluation of major diagenetic processes affecting the main hydrocarbon-producing interval of the Triassic Kurrachine Dolomite, in the central part of Syria, has proved to be of importance in understanding its reservoir characteristics. To describe the paragenetic sequence, core samples from two wells were studied by conventional petrographic analysis. In the first stage, pervasive replacement dolomitisation occurred. The dolomite petrography can be best explained by evaporative dolomitisation in the case of tidal flat facies and by the hypersaline brine reflux model in the case of subtidal facies. During relatively longer periods of subaerial exposure dolocrete sequences were developed. Dolomitisation of subtidal sediments in a more open-marine setting was completed during further burial by rising temperature. As a result, pervasively dolomitised rocks, with minor intercrystalline porosity in subtidal deposits and supposedly in dolocretes, were formed. In the deep burial realm, solution seams and stylolites acted as conduits for migrating hydrocarbon. Overmature hydrocarbon was observed as insoluble residue. Following fracturing and leaching by sulphate-bearing warm fluids, significant vuggy porosity was created but was subsequently occluded by precipitated anhydrite cement. The effective vuggy and fracture porosity of the Kurrachine Dolomite was created by latest stage leaching, which was controlled by the previous deep burial diagenetic processes and fracturing. The selective leaching, on the evidence of dissolution of anhydrite cement, improved the reservoir potential. Taken all together, the Kurrachine Dolomite exhibits evidence of important porosity development in burial conditions. Accordingly, the analysed core porosity and permeability data within the formation varies by location.

Diagenesis of the Zechstein Ca-2 carbonate from the Løgumkloster-1 well, Denmark

The carbonate unit Ca-2 found in the Løgumkloster-l well is located on the northern marginal carbonate platform of the North German Zechstein Basin. The ea. 43 m thick sequence includes former oncoidal/algal muds which were deposited in a relatively quiet lagoonal back-barrier environment, and ooidal carbonate sands, deposited in a rather agitated shoal environment. The carbonate sediments of Ca-2 have been subjected to a complex sequence of diagenetic events. However, the present porosity/ permeability, and thus the reservoir quality of the rocks, is primarily linked to four events: two leaching phases, a phase of chemical compaction, and a late anhydritization. The first phase of leaching created mouldic porosity. The last phase of leaching was important as it resulted in a widening of the preexisting pores and fractures and thereby facilitated percolation of sulphate solutions causing the late anhydritization, which considerably reduced the porosity/ permeability of the rock. The present pore geometry of the rock is complex. In the oolitic intervals the intra- and inter-ooidal porosity types are the most widespread. In the oncolitic/algal intervals intra- and inter-oncoidal porosity and vuggy porosity are often combined with intercrystalline porosity. The corresponding Ca-2 rock in the Brøns-l well, situated ca. 20 km northwest of the Løgumkloster- l well, was subjected to the same post-depositional evolution. Comparison with the diagenesis of the upper part of the Ca-la formation in the Aabenraa-1 well and the Ca-1 formation of the Varmes-1 well also shows close similarity in the diagenetic evolution.

Dolomitization and dedolomitization in the Senonian chalk of Northern Ireland

SummaryA dolomite locality in the Senonian chalk at Mullaghcarton shows a sequence of fabric variations between the unconformity with the Trias and a discontinuity surface marking the top of the basal beds. The four main variations are (1) xenotopic dolomite at the base, (2) hypidiotopic dolomite with zoned crystals and a calcite sealed intercrystalline porosity, (3) zoned dolomite rhombs scattered in a microspar micrite matrix, and at the top (4) micrite with rhombohedral pores. The xenotopic fabric is interpreted as being due to distant source dolomitization and the hypidiotopic fabric with zoned crystals to local source dolomitization. The dedolomitization phenomena are related to the mechanisms of dolomitization; for distant source dolomitization the dedolomitization fabric mimics the dolomite fabric, whereas for local source dolomitization the dedolomite regenerates the pre-dolomitization fabric.