Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

摘要

Shallow-marine microporous limestonesaccount for many carbonate reservoirs. Their formation,however, remains poorly understood. Due to the lack ofrecent appropriate marine analogues, this study uses alacustrine counterpart to examine the diagenetic processescontrolling the development of intercrystalline microporosity.Late Miocene lacustrine microporous micrites of theMadrid Basin (Spain) have a similar matrix microfabric asCenomanian to Early Turonian shallow-marine carbonatesof the Mishrif reservoir Formation (Middle East). Theprimary mineralogy of the precursor mud partly explainsthis resemblance: low-Mg calcites were the main carbonateprecipitates in the Cretaceous seawater and in LateMiocene freshwater lakes of the Madrid Basin. Based onhardness and petrophysical properties, two main facieswere identified in the lacustrine limestones: a tight faciesand a microporous facies. The tight facies evidences strongcompaction, whereas the microporous facies does not.The petrotexture, the sedimentological content, and themineralogical and chemical compositions are identical inboth facies. The only difference lies in the presence ofcalcite overgrowths: they are pervasive in microporouslimestones, but almost absent in tight carbonates. Earlydiagenetic transformations of the sediment inside a fluctuatingmeteoric phreatic lens are the best explanation forcalcite overgrowths precipitation. Inside the lens, the dissolutionof the smallest crystals in favor of overgrowths onthe largest ones rigidifies the sediment and prevents compaction,while partly preserving the primary microporousnetwork. Two factors appear essential in the genesis ofmicroporous micrites: a precursor mud mostly composed oflow-Mg calcite crystals and an early diagenesis rigidifyingthe microcrystalline framework prior to burial.