Publication Date

2012-06-20

Availability

Open access

Embargo Period

2012-06-20

Degree Name

Master of Science (MS)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2012-04-13

First Committee Member

Gregor P. Eberli

Second Committee Member

Donald F. McNeill

Third Committee Member

Ralf J. Weger

Fourth Committee Member

James S. Klaus

Abstract

Laboratory measurements of petrophysical properties, including porosity, permeability, acoustic velocity, and electrical resistivity, were performed on over 150 plugs of Pliocene-Pleistocene reefal successions in the southern Dominican Republic. These reefal successions are intensively altered by multiple episodes of meteoric diagenesis. The petrophysical measurements can be related to their diagenetic overprint and the pore structure, which is quantified by digital image analysis. The inherent depositional heterogeneity in reefs combined with the dynamic meteoric diagenesis processes (cementation and dissolution) produce abundant heterogeneity in the petrophysical properties. Reservoir properties such as porosity and permeability range from 0.07 to 0.54 (porosity) and up to six orders of magnitudes from 0.01 milidarcy up to 2 darcy in permeability. Although permeability shows a weak positive correlation to porosity, the variation at any given porosity can be as high as 4 orders of magnitude. This variation is caused by the different pore types created during deposition and diagenesis. Samples altered in the meteoric vadose zone have higher permeability due to occurrence of connected vug porosity. The P-velocity (Vp) at 5Mpa effective pressure ranges from 2900 to 6137 m/s with deviations up to + 2000 m/s from Wyllie time average curve of calcite at any given porosity. The positive deviation is related to intense cementation during deposition and multiple episodes of meteoric diagenesis. The influence of these processes on the pore structure and Vp can be quantified by digital image analysis (DIA). At the same porosity, samples with larger (higher DomSize) and simpler (lower PoA) pore structure have a higher velocity than samples with a smaller (lower DomSize) and more intricate pore network (higher PoA). By assigning the pore structure parameters to velocity-porosity estimations, the prediction of porosity from acoustic data can be significantly improved to a R2 of 0.91. It can be concluded that the porosity in combination with pore size and complexity of the rocks are the most important controlling factors for the acoustic velocity in these reefal carbonates. Electrical resistivity is assessed by the formation factor and the cementation factor (m). The cementation factor shows a significant range with values between 2.1 to 5.3. It has a very weak correlation to permeability (R2=0.15), implying that a) permeability is not well correlated to electrical resistivity and b) permeability estimates from resistivity logs contain large uncertainties. Furthermore, no good correlation exists between electrical properties and depositional environments, diagenesis, pore types, or rock textures. However, a higher amount of microporosity correlates to lower resistivity corroborating previous findings that an increased number of pores decreases resistivity.

Keywords

Petrophysics; Reefal Carbonates; Acoustic Velocity; Electrical Resistivity; Porosity; Permeability

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