Abstract
AbstractThis study evaluates variations in petrophysical properties within a mixed alluvial to lacustrine carbonate system (Miocene, Madrid Basin, Central Spain). The transition from alluvial environments to lake margins settings displays a shift from alluvial, siliciclastic red sandstones and mudstones to palustrine–lacustrine mudstones to packstones. Fluctuations in lake‐water level enabled land plants to occupy the lake margins during periods of low lake levels. The palustrine carbonates include features like pseudo‐microkarst, nodular and mottled limestones; the lacustrine deposits include enlarged root cavities, desiccation cracks and channel bodies. Scarce fresh water biota comprises charophytes, gastropods and ostracods. The sediments possess high natural, irregular varying, gamma‐ray values at the alluvial–lacustrine transition, and low, but constant values at full lacustrine sites. Acoustic properties agree with lithological variations within individual facies. Porosity is the most important parameter influencing P‐wave and S‐wave velocities. The scatter in the velocity–porosity relationship links to the porosity type; macro‐porosity or microporosity. The wide range of pore types and pore sizes results in a weak porosity to permeability relationship for the carbonate‐dominated rocks with low permeability for microporous and high permeability for macro‐porous carbonates. The sandstones (probably only inhibiting interparticle porosity), and to a lesser extent the sandy wackestones to packstones, show quite a strong relationship between porosity and permeability. Elastic properties of mixed alluvial–lacustrine deposits (this study) and marine deposits (literature data) overlap as variations in pore structures and porosity values are similar. Only 16% of the marine and lacustrine carbonate sediments display Equivalent Pore Aspect Ratio values above 0.2. In travertine deposits, 83% of the samples exceed Equivalent Pore Aspect Ratio values above 0.2, which highlights that travertine deposits are dominated by larger Equivalent Pore Aspect Ratios compared to lacustrine and marine carbonate deposits. Travertines display other rock frameworks with different pore types, pore distribution and amount of porosity.
Highlights
IntroductionTextures of continental limestones differ in many cases from marine limestones, due to: (a) different carbonate components (bioclasts), (b) frequent exposure periods (low lake levels), (c) common stable mineralogy of continental limestones and (d) different operating diagenetic processes, amongst many others, these sediments probably will have different petrophysical properties
Textures of continental limestones differ in many cases from marine limestones, due to: (a) different carbonate components, (b) frequent exposure periods, (c) common stable mineralogy of continental limestones and (d) different operating diagenetic processes, amongst many others, these sediments probably will have different petrophysical properties
The lacustrine deposits forming the Intermediate unit in the Madrid Basin were deposited in a shallow fresh-water lake, with detrital sediments interfingering with carbonate lake sediments at the edges of the lake
Summary
Textures of continental limestones differ in many cases from marine limestones, due to: (a) different carbonate components (bioclasts), (b) frequent exposure periods (low lake levels), (c) common stable mineralogy of continental limestones and (d) different operating diagenetic processes, amongst many others, these sediments probably will have different petrophysical properties. Palustrine limestones are a distinctive type of lacustrine limestone that respond to the modification of carbonate-rich muds by wetting and drying under subaerial exposure (Armenteros et al, 1997, 1998; Freytet & Plaziat, 1982; Plaziat & Freytet, 1978). They are common in relatively flat and low-energy lakes and slight variations in lake level can cause the exposure of significant lake areas (Abels et al, 2009; Alonso-Zarza, 2003; Alonso-Z arza et al, 2014; Freytet & Plaziat, 1982)
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