Author:

Abstract:

In 2006 the Lula oil field was discovered, with Pre-Salt continental carbonates acting as reservoir rocks. Similar reservoir discoveries, in front of the east coast of South America as well as west of Africa, resulted in an increased interest in these continental carbonates. Several of the observed core fabrics are widely recognized in travertines. Furthermore, seismic data reveal domal progradational-aggradational carbonate build-ups and in the Namibe Basin (Angola) fracture fed moundlike morphologies are linked to fissure ridge and “dam and cascade” travertines. From an exploration point of view the distribution and extension of different geobodies and their petrophysical properties need to be delineated. However sedimentological observations in discovered reservoirs are limited to core scale and should therefore be extrapolated to a larger context. Hence the necessity for travertine analogue studies worldwide. A sedimentpetrological investigation was executed on travertines of the Ballık area (Denizli Basin, Turkey). A 3D depositional model was constructed in combination with an extensive sedimentological description from fabric to geobody scale. The petrophysical properties are determined with helium porosimetry and nitrogen permeability measurements, mercury injection capillary pressure, water saturation, nuclear magnetic resonance, acoustic velocity and computed tomography. The large-scale depositional travertine system can best be compared to a mound or fissure ridge, with the spring(s) controlled by tectonic activity within the Denizli Basin. The travertine build-up can be split up into two main systems. The first system, represented by the sub-horizontal and biostromal reed travertines, was formed in a shallow dominantly sub-aquatic environment. The second travertine system, mainly represented by the sloping facies, formed in a thin water film in a dominantly sub-aerial setting. A general progradation of the system is observable due to the occurrence of the stacked sloping travertines, resulting in sigmoidal clinoforms with downlap terminations. The question could be raised how these travertines would look on a seismic line. They would appear with an external mound morphology, associated to normal faults in an extensional setting. Contacts with other lithologies will be easily recognisable due to their different acoustic impedance. The external morphology of the mound will thus result in strong reflectors. Inside the mound, the biggest acoustic impedance contrasts will be caused by differences in porosity and pore types. The lithofacies distribution will thus dominate the internal structure. Particularly waterfall travertines (with primary caves), when present, should be observable. They would result in a progradational pseudo-seismic line in an angle to the contacts of the sloping with the sub-horizontal facies. The size, morphology, and extensional tectonic setting are in good agreement with the aggradational-progradational carbonate build-ups reported for the Pre-Salt play of the South Atlantic. The presented lithofacies geobody model in combination with petrophysical and acoustic data can thus possibly form the basis for a reservoir analogue and synthetic seismic modeling of the Pre-Salt carbonates and other domal travertine reservoirs worldwide.