The use of ichnofossils in geological and petrophysical characterizations of aquifers and reservoirs: Examples from south-central Texas and southeast Utah

Abstract

This dissertation presents three studies that show ichnological assessment is a vital part of both hydrogeology and petroleum geology. Ichnological assessments lead to a better understanding of the effects of bioturbation on aquifer or reservoir quality and consequently on subsurface fluid pathways. Ichnofossils may be studied in both outcrop and core, making them easily accessible to most studies. Ichnological assessment has become increasing common in petroleum geology but is still overlooked in hydrogeology. The Trinity aquifer is a primary source of water for the San Antonio and Austin metropolitan areas. The Lower Cretaceous (Aptian–Albian) Lower Glen Rose Limestone (GRL) contains the middle Trinity aquifer and has previously been subdivided into six hydrostratigraphic units (HSUs). The GRL is a dual-permeability system and fluid flow is directed through both solution-enhanced fractures and pervasive Thalassinoides networks. Faults and fractures in the region are generally vertical and allow meteoric water to enter the subsurface. Thalassinoides-networks are commonly filled with coarser sediment than the surrounding matrix and act as lateral fluid pathways between fractures. Strata with well-developed burrow networks (ii3–4) are the most transmissive. Heavily bioturbated beds (ii5–6) are homogenized and restrict fluid flow. Beds with little to no bioturbation (ii1–2) can transmit water only through intergranular permeability, which is generally low. The GRL HSUs were first identified in the subsurface via core examination at the Camp Stanley Storage Activity (CSSA) and were then correlated to associated gamma-ray and resistivity logs. Resistivity logs show that resistance values greater than 300 Ω-m correlate with well-developed biogenic porosity (ii3–4) and values greater than 650 Ω-m are associated with solution enhancement of the Thalassinoides networks. These high resistivity zones are cyclical and are identified in confining units, in the absence of karstic development. Natural gamma-ray logs are inversely correlated to resistivity logs and can be used to correlate lithology. Combining resistivity and natural gamma-ray datasets allows for the subsurface correlation of GRL fluid pathways. Ichnologic assessment is not limited to physical properties and can be used for refined paleoenvironmental and paleogeographic histories. The Pennsylvanian–Permian (Virgilian–Wolfcampian) Halgaito Formation (HF) is a succession of carbonate and siliciclastic strata in southeastern Utah. The HF has been the subject of differing paleoenvironmental interpretations by various authors. This study refines the depositional history of the HF using a combined ichnological, paleopedological, and sedimentological approach. This study indicates that the retreat of the Elephant Canyon seaway out of the Paradox basin was punctuated by at least four transgressions. Above these transgressive units paleosol development generally increases upsection and ichnofossils suggest better-drained conditions. The uppermost beds of the HF contain little paleosol development and few ichnofossils, indicating a transition to more arid conditions prior to the deposition of the overlying Cedar Mesa Formation.