Permo-Carboniferous Glacial Sandstone Evaluation - Heavy Minerals in Glacial Sandstone and Effects on Thermal Neutron Porosity
Authors: Khalifa Aqeel , Almakrami Ibrahim , Clerke Edward , Oraby Moustafa , Soua Mohamed , Craigie Neil , Scheibe Christian 
Published: November 2018

Glacial continental sandstones are much less frequently encountered as hydrocarbon reservoirs than their fluvial and marine sandstone counterparts. Sands transported by water are subject to grain size fractionation during the transport process while glacial sandstones are not. Glacial sandstones are commonly very poorly sorted, ranging from silts to boulders, and neither size sorted nor density sorted. Hence glacial sands may also contain dispersed heavy minerals that are not removed by aqueous transport processes or by long lateral fluvial transport. The provenance of glacial sediments reflects components sourced from their high altitude origins which are mountainous terrains with igneous and/or metamorphic rocks. For similar reasons, glacial sands are commonly bereft of primary shale and clay. The formation evaluation starting point for glacial and reworked glacial sandstone analysis is therefore quite different from the more commonly encountered fluvial and marine sands. Instead of starting with a sand – clay or sand - shale analysis, we start with a sand – heavy mineral analysis.

Lower to middle Unayzah Group siliciclastic sediments in central and eastern Saudi Arabia represent a major glaciation and deglaciation of Gondwana. The glacial sediment origins and proximity to igneous and metamorphic outcrops ensures enrichment, preservation and recycling of heavy minerals such as zircon, monazite, apatite, rutiles and sphenes as observed by Knox and coworkers. Such minerals are strongly associated with the presence of rare earth elements (REE). REE's can be strong neutron absorbers and their variable presence in glacial and glacial reworked sandstones complicates the sandstone formation evaluation. These variations are anticipated to drive an important and variable sand matrix correction to the thermal neutron well log porosity for Unayzah Group sandstones.

Gadolinium (Gd), is the strongest contributor to the neutron absorption cross section of the REE group. Conventional thermal neutron well log interpretation of sandstone assumes that the dominant neutron processes are related to the presence of chlorine and hydrogen in the pore space fluids and in formation clays. These are not the only contributors in this case. If undetected, the REE presence can affect the porosity and matrix determination by neutron-density crossplot and thus affect the effort to use the apparent hydrogen index for distinguishing gas from oil.

Geochemical and petrographic studies of the Lower to middle Unayzah Group sediments demonstrate the significant presence of REE within the glacial/deglaciation complex where fine silts and boulders are equally transported within and by glaciers and deposited in distal parts of the basin. The use of neutron and density log data to estimate porosity is valid in most fluvial-deltaic sandstone reservoirs but this does not hold true for the heavy mineral bearing sands of the Unayzah Group.

In this work, the evidence for REE presence in Unayzah sandstones is reviewed using an extensive (tens of thousands) cores and cuttings samples database analyzed for elemental composition by inductively coupled plasma-mass spectrometry (ICP-MS) and optical-emission spectrometry (ICP-OEs) for 50 elements. Analysis of these data demonstrate that total REE's in Unayzah sediments are directly proportional to the lab-measured and wireline measureable Gadolinium (Gd) concentrations. These insights combine to indicate a significant relationship between the neutron capture cross section of the formation matrix and the geochemical well log detection of Gd, and thereby, insights into an improved method for the use of the neutron well log in the formation evaluation of the Unayzah Group.