Diagenetic phenomena in Jurassic-Cretaceous limestones of an area in the Huimanguillo-Comalcalco-Alto de Jalpan basins and first observations from scanning electron microscope and 3D tomography

The petroleum systems of the Southeast basin (Tabasco, Mexico) are characterized by having source rocks mainly from the Upper Jurassic, Cretaceous storage rocks, and Upper Cretaceous to Cenozoic seal rocks. In general, most of these reservoirs reflect a complex diagenetic history that depends on the prevailing porosity and permeability. A new approach using tools such as scanning electron microscopy (SEM) and X-ray 3D microtomography contribute to the understanding of how hydrocarbons accumulate and flow even at low permeabilities. The rock specimens observed by means of SEM are strongly altered by diagenetic processes of corrosion-dissolution-precipitation, which generated fractures, dissolution, stylolites, vug lining, decarbonatization, and dedolomitization. The paragenetic succession of such diagenetic processes can be schematized thus: dolomite(1) -> calcite(1) -> quartz(1) -> pyrite -> dolomite(2) -> calcite(2) -> quartz(2) -> calcite(3) -> quartz(3) -> chlorite -> zeolites -> clay minerals -> celestine -> calcite(4) with hydrocarbons. It is worth noting the widespread presence of quartz and the partial replacement of some fossils by framboidal pyrite. The latter, along with clays and zeolites usually occupy spaces left after the corrosion of dolomite crystals, and also occur within vugs and fractures, as an intermediate to late stage in the paragenetic sequence. The latter is characterized by the following stages: (1) cementation of carbonates, (2) thermobaric alteration, associated with the incursion of fluids that were characterized by means of microthermometry of fluid inclusions: a. in dolomite, with salinities that range between 12 and 19 wt. % NaCl equiv., and temperatures of homogenization (Th) between 99 and 141 degrees C, b. in diagenetic quartz in vugs and fractures, with salinities that range between 12 and 15 wt. % NaCl equiv., and Th between 112 and 131 degrees C, c. in the earliest calcite, with salinities that range between 12 and 14 wt. % NaCl equiv., and Th between 110 and 149 degrees C, (3) a late stage of corrosion. New information obtained by means of X-ray microtomography reveals that most of the vugs are not interconnected, and that the samples have abundant fossils randomly distributed in the micritic matrix and oriented according to the stratification. Microfractures and nano-pores in the matrix are partially responsible for the development of the vugs where cavities are formed after corrosive solutions that initially generated micro-breccias, followed by a partial lining by quartz and calcite.