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 δ¹³C values (−2.4‰ to +1.7‰) are derived by dissolution of host rock, and their δ¹⁸O values (−6.3‰ to −3.7‰) 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 occurred during stage 2 precipitation, causing the change from fibrous to equant crystal fabric. The δ¹⁸O_PDB values of stage 2 calcite (−3‰ to −9‰) are linked to precipitation from an ¹⁸O-enriched fluid that evolved to ∼ +6‰ _SMOW at temperatures of 60°C to 120°C, using the equation of Kim and O'Neil (1997).

Many open macropores are lined with small cement crystals that CL shows have all stages of growth present and are terminated by euhedral surfaces. These macropores are only partly filled because of cement inhibition, not because they were dissolved during burial, as was previously claimed. Late-stage dissolution did create some macropores and rounded micrite crystals, but its effect on overall platform porosity is small.

The CL examination of Tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without CL. The new Tengiz diagenetic textures recognized here are probably present in other carbonate platforms, especially those from icehouse epochs.