Preservation of carbon isotopes in kerogen from thermally altered Mesoproterozoic lacustrine microbialites

Stable carbon isotope geochemistry is a well-established and reliable tool for studying metabolisms of microbial communities in the Precambrian record; however, the isotopic effects of high-temperature alteration from igneous intrusions (i.e., contact metamorphism) have not been thoroughly explored. The Mesoproterozoic (∼1.4 Ga) Middlebrun Bay Member of the Rossport Formation, Sibley Group, in Ontario, Canada, is composed of carbonaceous stromatolites and microbial laminites preserved in an evaporitic, lacustrine chert–carbonate deposit and is cross-cut by an intrusive mafic sill at the studied locality. Sedimentary organic matter (kerogen) was investigated along two vertical stratigraphic transects to determine the spatial variability of its geochemical preservation. Thermal alteration of the preserved kerogen (as measured by Raman spectroscopy) increased toward the mafic sill, but the alteration was greater for kerogen preserved in carbonate mineralogies compared to that preserved in quartz (chert). Bulk δ13Corg values fluctuate throughout each vertical section, with a total average of −28.2‰ ± 0.8‰; however, values are unexpectedly lower for samples near the mafic sill, approaching −30‰, inconsistent with previously reported patterns. These measurements indicate that thermal alteration of sedimentary rocks does not universally result in 13C enrichment and increased δ13Corg values and suggests that ancient kerogen may be preferentially shielded from postdepositional heating effects due to micrometre-scale differences in mineralogy.

Challenge of Water Saturation in Low Resistivity Cretaceous Reservoirs

This study presents causes and reasons for lowering resistivity logs in carbonate deposit. Moreover this abstract elucidate methods for relieving of challenge water saturation estimation in cretaceous carbonate deposits with Low Resistivity Pay, in Persian Gulf. Reservoirs in the Cretaceous like Zubair, Buwaib, Shuaiba and Khatiyah formations of Southern fields have been analyzed as low resistivity carbonate. Resistivity responses reach less than 6 and even less than 1 ohm.m. Significant hydrocarbon accumulations are “hidden” these Pay zone, (LRPZ). Experimental analysis shows that reservoirs contain clay-coated grains of Lithocodyum algal and is along with Micrtization, Pyritization of digenetic process are reasons for effect on resistivity response. On the other side Smectite and Kaolinite of main clays types have high CEC and greater impact on lowering resistivity. Lønøy method applied to address pore throat sizes which contain Inter crystalline porosity, Chalky Limestone, Mudstone micro porosity. NMR (Nuclear magnetic resonance and Pulse Neutron-Neutron logs have been used to modify the calculated water saturation of the wells. The study shows that reduced specific resistivity is due to texture change and presence of microscopic porosity. For defining reliable water saturation, Core NMR and Log NMR results have been used. NMR results explain that decreasing of resistivity in pay zone is related to texture and grain size variation not being existence of moved water. Irreducible water for the reservoirs is estimated between 30 to 50 %. Low resistivity zone related to microspores with less than 3 micron. Variable T2 cut off is allows to choice suitable T2 cut off values to differentiate movable from bound fluids adapted for the specific carbonate rock. T2 cut off varies between 45 to 110ms. The proper T2 cut off for these formations are extremely crucial to being able to estimate permeability and water saturation.