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About this book

This book discusses how sediments compact with depth and applications of the compaction trends. Porosity reduction in sediment conveniently indicates the degree of sediments compacted after deposition. Published empirical curves- the compaction curves- are depth-wise porosity variation through which change in pore spaces from sediment surface to deeper depths e.g. up to 6 km can be delineated. Porosity is derived from well logs. Compaction curves, referred to as the Normal Porosity Profile of shales, sandstones and shale bearing sandstones of different models are reviewed along with the different mechanical and chemical compaction processes. These compaction models reveals how porosity reduces depth-wise and the probable reason for anomalous zones. Deviation from these normal compaction trends may indicate abnormal pressure scenarios: either over- or under pressure. We highlight global examples of abnormal pressure scenarios along with the different primary- and secondary mechanisms. Well logs and cores being the direct measurements of porosity, well log is the only cost-effective way to determine porosity of subsurface rocks. Certain well logs can detect overpressure and the preference of one log above the other helps reduce the uncertainty. Apart from delineation of under-compacted zones by comparing the modeled- with the actual compaction, porosity data can also estimate erosion.

Table of Contents

Frontmatter

Chapter 1. Compaction of Sediments and Different Compaction Models

Abstract
Various simple and advanced models exist for mechanisms of uniform and non-uniform sediment compaction that increases density and reduces porosity. While the classical Athy’s relation on depth-wise exponential reduction of porosity is not divided into any distinct stages, the Hedberg’s model involves four stages. Weller’s model utilized Athy’s and Hedberg’s relations to deduce a sediment compaction model. Power’s compaction model additionally considers clay mineralogy. Several other porosity/compaction models exist, e.g., those by Teodorovich and Chernov, Burst, Beall, and Overton and Zanier. The geometry of the depth-wise porosity profile depends on the sedimentation rate, compaction mechanism and pressure solution model. This chapter reviews porosity variation with depth for the following rock types: shales, shaly sandstones, sandstones and carbonates.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 2. Porosity in Carbonates

Abstract
Porosity in carbonate rocks, most commonly limestones and dolostones, is of great importance to study since around half of world’s hydrocarbon reserves are made up of dolomite and limestone, which formed mostly in a shallow marine environment and usually close to where such sediments originate from the source rocks. Carbonates possess both primary and secondary porosities, which reduces with progressive burial leading to increasing rigidity of the rock. Several classifications of carbonate rocks are available. These are based on texture, depositional environments (the three kinds of carbonate factories), energy of the depositional environment, mud to grain ratio (volume-wise), grain to micrite ratio, porosity-permeability parameters, depositional-, diagenetic- and biological issues etc. Out of them, those by Folk and Dunham have been entered most of the text books on sedimentology. Carbonates more commonly display dissolution, cementation, recrystallization and grain replacement than the siliciclastic deposits. The porosity-permeability relation in carbonates may or may not be linear. Several schemes of classification of porosity of carbonates are available. Archi’s scheme (based on qualitative evaluation of texture and porosity), the Choquette-Pray scheme (utilizes depositional and diagenetic changes in the rock), the Lucia scheme (works on inter-relationship between porosity, permeability and the particle size) etc.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 3. Pore Pressure Determination Methods

Abstract
Overpressure situation can be created in both clastic and non-clastic reservoirs when at some depth the formation pressure exceeds what is expected for a hydrostatic (normal/lithostatic) pressure scenario. Likewise an underpressure situation has also been reported from reservoirs after sufficient hydrocarbons have been extracted. Over- and underpressure can develop by both tectonic (e.g., horizontal or vertical stress) and atectonic processes (e.g., mineral phase change, kerogen maturation). Presence or withdrawal of water (saline and freshwater) and hydrocarbon can produce over- and underpressure. Fracture pressure and its gradient are important in planning well-drilling programmes. Pore pressure estimation has become an active field of research in the present day oil industry and several methods exist for such estimation.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 4. Detection of Abnormal Pressures from Well Logs

Abstract
Continuous attributes through depth are obtained using wireline logs and logging while drilling. A number of well logging techniques enables detection of overpressure zones. How porosity link with pore pressure is the main key to detect the abnormal pressure. Abnormal pressure, i.e., overpressure and under pressure, can be quantified by noting how much the depth-wise log-data for a rock type varies from that of a shale. Sonic logs can better detect abnormal pressure zones than the neutron and the density logs. Effective stress reduction opens connecting pores easier than the storage pores. This chapter explains pore pressure mechanisms using cross plots of wireline logs.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 5. Global Overpressure Scenario

Abstract
Dickinson made a serious attempt to understand the geoscience of pore pressure from his studies in the Gulf of Louisiana. Compaction disequilibrium has been considered classically as the mechanism of overpressure. Subsequently, thermal effects, clay and organic matter transformations and osmosis are the other mechanisms put forward by the authors for overpressure. This chapter reviews worldwide overpressure scenarios—from different continents, countries, (hydro-carbon bearing) rocks of different ages, structures, depths and tectonic and sedimentary regimes.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 6. Investigation of Erosion Using Compaction Trend Analysis on Sonic Data

Abstract
Compaction trends of sediments can decode the mechanism of compaction. Not all kinds of log detect all types of porosity, For example, while Neutron-, sonic- and density logs can decipher porosity, sonic tool cannot detect secondary porosity. Tectonic and isostatic uplift affect petroleum system. The Velocity-depth data from different terrains has been used in studying erosion of petroliferous basins. Porosity-depth trends in well data can indicate the amount of eroded sediment layer. How different authors estimated the thickness of the eroded overburden following different principles is discussed in this chapter.
Troyee Dasgupta, Soumyajit Mukherjee

Chapter 7. Pore Pressure in Different Settings

Abstract
Before 1980s geoscientists believed the main reason of overpressure was drilling-induced. Subsequently other processes, e.g., compaction disequilibrium and aquathermal expansion, responsible for overpressure were established. This chapter reports overpressure condition from two kinds of plate margins from several places in the world: (i) collisional margins especially at the accretionary prisms, subduction zones and decollement zones; and (ii) extensional margins such as rifts and passive margins where growth faults boundaries of continental shields are the zones of overpressure.
Troyee Dasgupta, Soumyajit Mukherjee
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