Please join us for two graduate student seminars this Friday January 18

Please join us for two graduate student seminars this Friday January 18 in rm 155 Geology, presented by:

3:30 pm

Fernando Valencia, MSc candidate

DIAGENESIS OF THE PERLA LIMESTONE, GULF OF VENEZUELA BASIN: IMPLICATIONS ON THE PETROPHYSICAL PROPERTIES

The Perla limestone is an Oligo-Miocene carbonate reservoir located in the Gulf of Venezuela Basin. This reservoir is considered the largest gas field in Latin America, and the largest offshore gas field in the Western Hemisphere, with more than 20 trillion cubic feet (TSCF) of gas in place. The Perla limestone shows a complex porosity system marked by a strong diagenetic control. Despite comprehensive depositional facies modelling carried out in this reservoir, inconsistencies remained when distributing petrophysical properties with depositional facies. Specifically, in areas strongly affected by diagenetic processes since they constitute a complex variable to reproduce in a predictive reservoir quality-model. In order to understand the impact of diagenesis in the Perla limestone, and its distribution along the carbonate succession; a detailed petrographic, mineralogical and chemical analysis on well-cores was performed. As a result, several diagenetic processes that created and destroyed porosity and permeability were identified and grouped chronologically in function of the evolution of the diagenetic environment. Within these processes, an important burial dissolution process, hypothetically linked to the ascent of CO₂-rich fluids mediated by basement-root faults and fractures allowed the development of pervasive secondary porosity in the vicinity of the major discontinuity surfaces. On the other hand, blocky-calcite cementation in the near surface to shallow burial environment, and the progressive mechanical and chemical compaction in the burial realm, were the main responsible for reservoir quality destruction. Contrasting the petrophysical properties of the Oligocene and Miocene units of the Perla limestone, the Miocene units (with some exceptions) have better petrophysical properties, and this is mainly due to a combination of a greater impact of burial dissolution processes and primary-porosity preservation.

4:00 pm

Isabelle Baconnais, PhD Candidate

Investigation of chromium isotope variability in the Canadian Arctic Archipelagos

I. Baconnais^1*, C. Holmden¹, R. François²

¹ Saskatchewan Isotope Laboratory, University of Saskatchewan, Saskatoon, Canada (*correspondence: isabelle.baconnais@usask.ca; ceh933@mail.usask.ca)

² University of British Columbia, EOAS, Vancouver, BC, Canada (rfrancoi@eos.ubc.ca)

Measurements of total dissolved chromium concentration ([Cr]_T) and their isotopic ratio (δ⁵³Cr) in modern oceans has increased with the need to calibrate Cr as a proxy for paleo redox changes in the ocean and atmosphere. In 2015, a set of 20 stations were sampled along a transect from the Labrador Sea to the Beaufort Sea, by way of the Canadian Arctic Archipelago (CAA). Preliminary measurements of [Cr]_T and δ⁵³Cr range from 77 ng×Kg^-1 to 263 ng×Kg^-1, and –0.07 ‰ to +1.54 ‰ respectively. These waters are the first to plot systematically below the global Cr array on the δ⁵³Cr vs. ln(Cr) diagram, which, thus far, describes the behaviour of Cr and its isotopes in three of the worlds major ocean basins, including 47 new samples of waters from the subarctic Pacific down to 4200 m depth (see figure). The slope of the global Cr array is interpreted to reflect a globally unique fractionation factor associated with the dominant process(es) by which soluble Cr(VI) is reduced in the oceans to particle reactive Cr(III) species, which are susceptible to scavenging and export on sinking particles. Some of the exported Cr(III) is released into deep waters where the particles dissolve, and eventually re-oxidized to Cr(VI), thus, driving the deep ocean towards lower δ⁵³Cr values and higher [Cr]_T than the surface mixed layer.

In this talk, we will evaluate potential mechanisms which can explain why the redox-imprinted signature of dissolved Cr cycling seen elsewhere in the oceans is augmented in the Labrador Sea and CAA, with a particular emphasis on mixing effects involving local riverine inputs of Cr.