CHAPTER 1 GEOLOGIC CHARACTERIZATION AND EXPLORATION CONCEPTS APPLIED TO CONVENTIONAL AND RESOURCE BASE EXPLORATION PLAYS

• Jurisdiction: United States

Oil and Gas Agreements: The Exploration Phase
(Mar 2010)
CHAPTER 1
GEOLOGIC CHARACTERIZATION AND EXPLORATION CONCEPTS APPLIED TO CONVENTIONAL AND RESOURCE BASE EXPLORATION PLAYS
Renato Tadeu Bertani
CEO
Thompson & Knight
Global Energy Services, LLC
Houston, Texas

Renato T. Bertani holds a Ph.D. in Sedimentary Geology and Geochemistry from the University of Illinois, USA. He has more than 33 years of international experience in exploration and production projects, acquisitions and divestments in several Latin American countries (particularly Brazil, Colombia, Bolivia, Peru, Ecuador, and Argentina), West Africa, UK, and the USA Gulf of Mexico. Mr. Bertani took over in March of 2007 as President and CEO of Thompson & Knight Global Energy Services, LLC, a subsidiary of Thompson & Knight dedicated to render business development and portfolio management services to the energy industry. In this position, in addition to providing services to numerous petroleum companies, he has been serving as an independent member of the Board of Directors of Green Hunter Energy, Inc., a company focused on renewable sources of energy, based in Grapevine, Texas. He previously served for 31 years in various technical and high level managerial positions with Petrobras, the Brazilian state oil company. Among such positions were President of Petrobras America, Inc., a subsidiary of the Brazilian state company based in Houston (2001-2006), Managing Director of Petrobras UK (1998-2001) and Director of International E&P activities for the Petrobras Group (1992-1997). Mr. Bertani is Vice President of the World Petroleum Council, responsible for the technical program for the next World Petroleum Congress that will take place in Doha, Qatar, in 2011. He also serves on the advisory board of the Center for International Studies of the St. Thomas University. He served as a trustee of the Museum of Fine Arts of Houston from 2003 to 2006, as President of the Brazil Texas Chamber of Commerce from 2002 to 2007, and as Director of the Brazilian Petroleum Institute from 1995 to 1997.

1. Introduction

For over a century oil and gas exploration has been focused on traditional plays characterized by the combination of certain geologic features that, when present and properly mapped by the explorationists, greatly enhanced the success probability of exploratory drilling. It was mostly in the last decade that the so called resource base plays became a technically and commercially viable alternative to finding and developing oil and gas reserves.

In this paper the geologic elements that are necessary for the existence of economically attractive accumulations of oil and gas in both types of plays are discussed and compared. The combination of such elements in a certain region or sedimentary basin is usually referred to as the petroleum system of such province.

2. Origin and Evolution of Sedimentary Basins

The vast majority of the know petroleum reserves are contained within sedimentary basins, which consist of thick sequences of mostly sedimentary rocks deposited in depressions of the Earth's crust. In most cases these crust depressions, or basins, were occupied by a body of water, like lakes, seas or oceans, and then gradually filled by sediments that can exceed 10km in total thickness.

The Earth's crust consists of an extremely thin (compared to the Earth's diameter) veneer of brittle rock, with total thickness ranging from a few kilometers to about 75km, underlain by a more ductile, hotter and denser layer of rocks known as the mantle. Continents are essentially made up of granitic type rocks (Continental Crust), while ocean floors are made up of basaltic type rocks (Oceanic Crust)

Compositional Layers

The Earth Crust consists of a very thin skin of solid, brittle rocky materials.

The Continental Crust is made up of granite-like (10-70km thick) The Oceanic Crust is made up of basalt-like rocks (5-7km thick).

Mantle: made up mostly of Iron and magnesium silicates. The Upper Part of the Mantle is ductile and flows (although being solid) and is called Aesthenosphere.

Core: composed of an Iron and Nickel alloy. The Outer Core is liquid, while the Inner Core, at temperatures of ??

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There are three main processes that lead to the formation of basins:

— Stretching and thinning of sections the crust

— Cooling and subsidence of sections of the crust

— Isostatic compensation to overload and subsidence

Even though all three mechanisms may be active at the same time, usually basins evolve through well defined patterns while the prevailing genetic cause for their formation changes from crustal thinning to thermal and then isostatic subsidence.

The tectonic plate and continental drifting theory provides the unifying framework that neatly explains how basins form and evolve throughout the geologic times.

Basins start to be formed when continents break apart and form huge rift systems while the continental crust is being stretched and thinned. A good example of this phase of the life of a basin is provided by the African Rift Valley System, a series of elongated lakes that vary from shallow to very deep, filled with water that may be fresh all the way to extremely salty and alkaline brines.

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As the plates move apart from each other the rift valley lakes evolve to narrow gulfs, like the Red Sea, and then to open oceans. At the same time, upwelling of basaltic lavas create new crust that fills in the gap between the continental masses drifting apart. In this stage subsidence is mostly driven by thermal cooling of the stretched continental crust and new an hot oceanic crust.

Finally, stretching and cooling give way to a lower rate of subsidence, controlled by isostatic adjustment and downward bending of the crust in response to the overload of the sedimentary section. In some situations, when massive overloads are built (volcanic constructions) or thrust (mountain ranges) onto the crust, the isostic subsidence can actually be the triggering and prevailing cause for the formation of deep basins.

3. Filling of sedimentary basins

At any moment in their evolution the extent to which sedimentary basins are filled is essentially dictated by the balance between the rate at which sediments are brought in or formed in situ, and the rate of subsidence. These two factors determine, to a large extent, the geometry and composition of the sedimentary layers that ultimately fill these depressions up.

Sedimentary rocks can be classified in various ways, one of which is in accordance with the origin and nature of the sediments, as follows:

1) Allochtonous or terrigenous sediments: these are sediments brought into the basin by rivers, currents, winds and sometimes glaciers. Sedimentary rocks thus formed are classified in accordance with the size of the majority of individually transported grains:

— Conglomerates: 〉 2mm in diameter, most commonly consisting of a mixture of clays, small grains to pebbles to large size rock debris;

— Sandstones: 2mm to 1/16 mm: usually consist mostly of quartz grains, and secondarily of other minerals observed in igneous or metamorphic rocks, such as feldspars and micas;

— Siltstones: 1/16 mm to 1/256 mm: also consist mostly of fine quartz grains

— Shales: 〈 1/256 mm formed mostly by clays which are derived through weathering of older rocks. Clays are usually transported in suspension and settle down once reaching a calm, low energy body of water, usually forming orderly layers in the more distal and deeper parts of the basins.

2) Authochtonous or in situ sediments: these are sediments formed inside the basin, of two main origins:

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— Biogenic: limestones are essentially formed in situ from the calcareous skeletons of various types of organisms that live in lakes and seas, such as clams, algae, zooplankton, corals, etc. Coarse grained limestones are formed by large size fragments while fine grained limestones are derived from tiny calcareous needles the breakdown after the death of certain types of algae and other microorganisms.

— Chemical: in arid climates and when the basin consists of shallow or narrow lakes or seas the water can become extremely salty and lead to the deposition of thick sequences of salt layers. The most common is our trivial kitchen salt (NaCl) but in extreme salinity situations other less common types of salt can form as well (KCl, KMgCl3).

3) Volcanic rocks: layers of ashes or lava flows, as well as tabular sills and dykes that intrude into the pre-existing sedimentary layers, are also very frequent in sedimentary basins.

Organic matter is also a conspicuous component in the filling of sedimentary basins. It also may be classified in accordance with the location of origin versus deposition:

— Allochtonous: mostly consist of debris of trees, ferns and other superior vegetation. In certain geologic times and conditions these formed thick layers in near shore depositional environments and, upon burial and thermal maturation, formed vast coal deposits;

— Autochtonous: mostly organic remains of microorganisms that live in the body of water (zooplankton and phytoplankton) which settle at the bottom of the basin when these organisms die. This type of organic matter, also known as kerogen, tends to be destroyed in well oxygenated, high energy water environment. Conversely, kerogen tends to be preserved and become more concentrated in the more distal and deeper parts of the basin, interspersed within sequences of shales and fine grained limestones. Kerogen is prone to generate oil in the early phases of thermal maturation and gas as its submitted to much deeper and hotter conditions.

4. Modifications of sedimentary layers

As the geologic basins evolve through time and phases of subsidence the sedimentary layers deposited inside them go through significant modifications, linked to two main processes: burial and tectonism.

a) Burial

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