2018 | Korneva, I., Bastesen, E., Corlett, H., Eker, A., Hirani, J., Hollis, C., Gawthorpe, R., Rotevatn, A. and Taylor, R.L.
Petrographic and petrophysical data from different limestone lithofacies (skeletal packstones, matrix-supported conglomerates and foraminiferal grainstones) and their dolomitized equivalents within a slope carbonate succession (Eocene Thebes Formation) of Hammam Faraun Fault Block (Suez Rift, Egypt) have been analyzed in order to link fracture distribution with mechanical and textural properties of these rocks. Two phases of dolomitization resulted in facies-selective stratabound dolostones extending up to two and a half kilometers from the Hammam Faraun Fault, and massive dolostones in the vicinity of the fault (100 metres). Stratabound dolostones are characterized by up to 8 times lower porosity and 6 times higher frequency of fractures compared to the host limestones. Precursor lithofacies type has no significant effect on fracture frequency in the stratabound dolostones. At a distance of 100 metres from the fault, massive dolostones are present which have 0.5 times porosity of precursor limestones, and lithofacies type exerts a stronger control on fracture frequency than the presence of dolomitization (undolomitized vs. dolomitized). Massive dolomitization corresponds to increased fracture intensity in conglomerates and grainstones but decreased fracture intensity in packstones. This corresponds to a decrease of grain/crystal size in conglomerates and grainstones and its increase in packstones after massive dolomitization.
Since fractures may contribute significantly to the flow properties of a carbonate rock, the work presented herein has significant applicability to hydrocarbon exploration and production from limestone and dolostone reservoirs, particularly where matrix porosities are low.
Keywords: Limestones; Dolostones; Textural and petrophysical properties; Fracture spacing
2018 | Aratman, C., Taylor, R.L., Hollis, C., Özkul, M., Swennen, R. | International Participant Sedimentology Workshop
This study is subject to figure out facies heterogeneity by the integrated approach of lithotype and fabric determined over Denizli continental carbonates (travertine or tufa-like deposits). Besides, the petrophysical characteristics, such as acoustic-wave velocities carried out under dry-saturated conditions, helium porosity and Klinkenberg permeability were correlated with these lithotypes.
Field analysis enables four different lithotypes to be identified based on the quantity of lime mud, carbonate and coated grains, the different types of skeletal organisms, and the volume of lamination. These lithotypes were differentiated: (i) microsparite wackestone, (ii) intraclast packstone, (iii) massive shrubby grainstone, and (iv) boundstone. In the (iv) boundstone lithotype, five further sub-lithotypes were identified: (iva) shrubby, (ivb) granular shrubby, (ivc) laminated, (ivd) coated reed, and (ive) phyto. Petrographical studies allowed for each lithotype or sub-lithotype to be identified by their characteristic sedimentary fabrics according to the packing and organization of lithic peloids and peloidal shrubs. The compressional-wave (Vp) and shear-wave (Vs) velocities of the studied carbonate lithotypes vary between 4192 m s-1 to 6077 m s-1, and 2207 m s-1 to 3873 m s-1, respectively. Effective porosity ranges between 2.78% to 36.54%, while nitrogen (N) gas permeability varies between 0.002 mD to 58060 mD. The Vp/Vs ratios were measured between 1.5 and 2.19. Dynamic elastic moduli that consist of bulk (K), shear (G) and young’s modulus (E) were calculated from measured acoustic-wave velocities and bulk density in given lithotypes. (i) Microsparite wackestone lithotype, (iva) shrubby and (ive) phyto boundstone sub-lithotypes are differently clustered in the plot of the elastic moduli versus porosity. (ive) The phyto boundstone differs from other lithotypes in terms of porosity and bulk modulus. As effective porosity increases, the elastic moduli linearly decrease in all lithotypes, with the exception of the (iva) shrubby boundstone, in which an increase is observed. The (i) microsparite wackestone lithotype, the (ivc) laminated and (ive) phyto boundstone sub-lithotypes display the best correlation between porosity and Vp due to the fact that they possess massive sparite, clotted to mesh-work and spherulite fabrics that are associated with the highest degree of cementation and dissolution. Consequently, this study highlights that differences in petrophysical parameters occur between lithotypes. In addition, a strong relationship between porosity and permeability (R2 ˃ 0.5) is formed in each lithotype in which an inversely proportional is generated between porosity and acoustic-wave velocities.
Keywords: acoustic-wave velocities, fabric, lithotype, permeability, porosity
2018 | Aratman, C., Taylor, R.L., Hollis, C., Mohammadi, Z., Özkul, M., Swennen, R. | 6th International Geologica Belgica Meeting
Continental carbonates have recently received a lot of interest due to the presence of large volumes of hydrocarbons in these strata, for example in the Santos and Campos Basin (e.g.Tupi, Carioca, Guara, and Lara Fields; Wright, 2012; Herlinger et al., 2017) and in the Namibe, Congo and Kwanza Basins, offshore Angola (Schröder et al., 2012; Saller et al., 2016). Travertine outcrops in the Denizli Basin as well as in Italy and Hungary serve as outcrop analogues since they present several similarities to continental carbonate reservoirs (Terra et al., 2010; Erthal et al., 2017) in terms of microfacies components. However, no clear relationship between travertine/tufa-like lithotypes and porosity and permeability has been established because these lithologies are made up of complex sedimentary fabrics containing lithic peloids, peloidal shrubs and micro- or macrophytes encrusted by calcite. The latter relate to shallow- or deep-circulated fluids with varying temperatures from 20 to 40ºC under low to high alkalinity and CO2 degassing (Ford and Pedley, 1996; Pentecost, 2005). This detailed study of sedimentary fabrics and lithotypes however allows to refine the relationship of continental carbonates in the Denizli Basin (Turkey) to petrophysical parameters, such as porosity, permeability and acoustic velocities. In addition, it aims to address facies heterogeneity at the basin scale, by defining lithotypes formed under varying environmental conditions as suggested by Lopez et al. (2017).
Helium porosity and permeability analyses as well as acoustic-wave velocity measurements under dry-saturated and ambient conditions were carried out on 165 cores, each up to 7 cm length and 2.5cm diameter. Field analysis identified three different lithotypes based on the quantity of lime mud, carbonate and coated grains, the different types of skeletal organisms, and the importance of lamination. The following lithotypes were differentiated: (i) microsparite wackestone, (ii) intraclast packstone, (iii) massive shrubby grainstone, and (iv) boundstone. In addition, in the boundstone lithotype, shrubby, granular shrubby, laminated, coated reed, and phyto sub-lithotypes can be differentiated. Petrographical studies allow discrimination of each lithotype or sub-lithotype using their sedimentary fabrics according to the packing and organization of lithic peloids and peloidal shrubs. The compressional-wave (Vp) and shear-wave (Vs) velocities of the studied travertine lithotypes vary between 4192 m/s to 6077 m/s, and 2207 m/s to 3873 m/s, respectively. They show a porosity ranging from 2.78% to 36.54%, and a permeability ranging from 0.002 mD to 58060 mD. The Vp/Vs ratio ranges from 2.19 to 1.5.Each lithotype indicates various dynamic elastic moduli including bulk, shear and young’s modulus that were calculated from the measured acoustic-wave velocities and bulk density because the link of geological parameters of the lithotypes to seismic properties helps to infer their complex micro facies properties from velocity/porosity data. The microsparite wackestone, shrubby and phyto boundstone lithotypes are differently clustered in the plot of the elastic moduli versus porosity Furthermore, with respect to porosity and bulk modulus, the phyto boundstone clearly distinguished from other lithotypes that have similar seismic properties. As porosity increases, the elastic moduli decreases in all lithotypes, however, it increases in the shrubby boundstone sub-lithotype. The microsparite wackestone and the laminated boundstone as well as phyto boundstone (sub-) lithotypes display the best correlation between porosity and Vp due to their massive sparite, clotted to mesh-work, and spherulite fabrics that are associated with the highest degree of cementation and dissolution. Consequently, this study points out that differences in petrophysical parameters occur between lithotypes. Furthermore, each lithotype exhibits a relationship between porosity and permeability (R2 ˃ 0.5) while having an inversely proportional relationship with acoustic-wave velocities.