Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan

Abstract

Predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization. The diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the Tengiz reservoir (, 16 km); the late Visean (Lvis13 cbs 900) and Bashkirian (Bash2.5 MSF 350) parasequences are separated by 8 million years.Syndepositional fibrous cement (. 10% rock volume) and aragonite skeletal molds developed in both parasequences. On subaerial exposure, brown soil-related calcite, and meniscus and pendant cements were emplaced in both parasequences.The application of CL petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved. The principal calcite cement shows a complex growth pattern by CL that is unreported from any other calcite cement. CL is also used to divide the main cement into stage 1, nonluminescent, and stage 2, orange to brown luminescent, calcite. Stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations. Stage 1 cement is patchily distributed at a millimeter scale; 28% to 40% in the Visean and 0.5% to 22% in Bashkirian grainstones. Stage 1 cement is more abundant at the center of the platform in the late Visean than at its margin. Abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform. This sweet spot is reinforced by the deposition of late-stage bitumen cement that occupies 8% rock volume at the margin of the platform but is almost absent at its center. Stage 1 d 13 C values (22.4% to +1.7%) are derived by dissolution of host rock, and their d 18 O values (26.3% to 23.7%) are compatible with precipitation from meteoric or mixed marine-meteoric water.Depositional micrite and micritic grains are transformed by dissolution-precipitation processes into porous microrhombic calcite not by Ostwald ripening. Microrhombic calcite is ubiquitous in the Tengiz icehouse, dispelling the myth that this fabric is restricted to greenhouse epochs.At , 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity; their transformed texture set up flow paths that controlled the distribution of later diagenetic products. Reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences, but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow.CL examination of stage 2 calcite that adds 0.5% to 16% to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures. A change from acute to equant habit ...

The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ18O and δ13C of early cements

“…The δ 18 O signal found for isopachous fibrous cements is in the range of the analyses already performed by authors on recrystallized fibrous cements (Tobin et al ., ; Christ et al ., ; Dickson & Kenter, ).…”

“…Moreover, because of the low accuracy of microdrilling, d 18 C and d 13 C results could reflect mixing with later burial overgrowths and cements. Oxygen and carbon isotope signatures have already been acquired for well-developed early cements such as microbially generated fabrics, botryoidal cements (Christ et al, 2012a), radiaxial and fascicular fibrous cements (Tobin et al, 1996;Christ et al, 2012a;Dickson & Kenter, 2014), pendant cements (Ronchi et al, 2010;Brigaud et al, 2014b;Dickson & Kenter, 2014) and scalenohedral cements (Godet et al, 2016). However, such measurements are limited to exceptionally well-developed early cements; they do not reflect all the variability of cement types, especially since similar morphologies may have different geochemical compositions and, above all, they do not inform on the intra-cement variability.…”

Process‐based Modelling of Syn‐depositional Diagenesis

“…The Tengiz field in Kazakhstan is an isolated carbonate platform of Devonian and Carboniferous age with a stratigraphic thickness of over 2km (Weber et al, 2003;Collins et al, 2006). Its internal reservoir property distributions were thought to be strongly influenced by diagenetic processes (Kenter et al, 2010) and the extensive influence of syn--sedimentary meteoric diagenesis was not identified until recently (Dickson & Kenter, 2014). The application of CARB3D+ to this platform has helped provide a broad prediction of the stratigraphic architecture, facies distribution and meteoric influence on this system within a spatial framework (Figure 3.16;Frazer et al 2014).…”

“…This relies on a workflow which involves inferring diagenetic process from product, and predicting distributions based on an understanding of controls on process (e.g. Dickson & Kenter, 2014). For example, petrographic and geochemical evidence may suggest a given phase of cement was precipitated close to the water table, and thus, by reference to models of the spatial configuration of modern water tables in carbonate environments we may predict a near--horizontal zone of cementation controlled by the paleo--position of sea--level.…”

Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform, Red Sea, NW Saudi Arabia

“…Many re search ers (e.g., Bathurst, 1975;Allan and Matthews, 1982;Tucker, 1986;Bishop et al, 2014;Swart, 2015) have highlighted the sen si tiv ity of metastable ar agon ite and high-Mg calcite (HMC) to the changes of pore wa ter chem is try. The me teoric en vi ron ment, in par tic u lar, plays a key role in the po ros ity de vel op ment in car bon ate res er voirs (e.g., Smalley et al, 1994;Bishop et al, 2014;Dick son and Kenter, 2014;Ma jor, 2014). How ever, such stud ies have en coun tered many dif fi cul ties in rec og niz ing the fea tures and es tab lish the im pact of me te oric diagenesis from petrographic ev i dence solely.…”