1.  The Oil & Gas Story

1.1.              Introduction

There are three types of rock types that are studied when considering the geological environment. They are no particular order:

  1. Sedimentary
  2. Igneous
  3. Metamorphic

The circumstantial evidence has suggested that hydrocarbons that are formed and deposited are strongly associated with sedimentary rocks. They are the result of altered organic material derived from microscopic plant and animal life. The organic material is converted to hydrocarbons by the culmination of several key factors and conditions.

1.2.              The Conditions

For the organic material to become hydrocarbons the organic material must be exposed and subjected to specific conditions.

In general the organic material must be present in an anaerobic environment, if not the material will simply decompose and be destroyed. Only when the anaerobic bacteria begins to act on the organic material will the environment become a reducing one that is needed to begin the process that leads to the accumulation of hydrocarbons.

Given the anaerobic environment the next two conditions that need to be met they are pressure and temperature. Largely these two conditions will be determined by the depth of deposition. This will determine the overburden which will determine the pressure exerted on the organic material.

It important to note that the final size of the hydrocarbon reserve will be determined by the amount of organic material initially deposited.

1.3.              Source Rock

This usually made up of the deposits of fine silts, clays and organic material. These were once ocean margins that would have been good anaerobic environments for the alteration of organic material. Such deposits usually go onto form shale rocks that leads geologists to believe that shale rock is a source of hydrocarbons. The best environments would be black shales originally deposited in a non-oxidizing, quiet marine environment.

1.4.              Migration

The hydrocarbons in some altered form migrate from the source rock through other more porous and permeable beds to eventually accumulate in reservoir rock. Migration does not mean that the alteration has stopped. The movement is the result of hydrodynamic pressure and gravity forces. The burial pressure is what forces the water and oil out, the water carries out the hydrocarbons and eventually the hydrocarbons establish an equilibrium with the other fluids in the reservoir.

1.5.              Accumulation

This will only occur when there is a trap which stops the migration and promotes the accumulation of hydrocarbons. However this is only possible if the reservoir rock is permeable.

1.6.              Trap

Traps are caused by geological processes that create irregularities in the subsurface strata which causes the oil and gas to be retained in a porous formation. The rock that form a barrier or trap are referred to as cap rocks.

This is the geological feature that allows for the migrating hydrocarbons to not continue its upward journey and accumulate. This is what physically keeps or traps the hydrocarbons underground.

1.6.1.   Anti-clinal and Dome traps

The rock layers were originally horizontal then folded upward into and arch or dome. The hydrocarbons migrated upwards later on only to be trapped by an impermeable shale.

The common characteristics is that the oil-water contact surrounds the hydrocarbon accumulation. This structure usually extends through formation of considerable thickness.

1.6.2.   Salt Dome or Salt Plug trap

The intrusion of stratified rock layers by a ductile non-porous salt. The intrusion causes the lower formations to be uplifted and truncated along the sides of the intrusion. The layers above are uplifted. Hydrocarbons accumulate alongside the salt dome if porous, permeable beds and a capable cap-rock exist.

1.6.3.   Fault Trap

This is the result of vertical and horizontal stress. At some point the stress causes the rock to break. The rock faces eventually become offset to each other. When a non-porous rock seals off a porous rock then the hydrocarbons have time to accumulate.

They depend on the effectiveness of the seal at the face of the fault. Fault trap accumulations tend to be elongated and parallel to the fault trend.

1.6.4.   Stratigraphic Traps

Result of differences between or within stratified rock layers. The end result is a change in permeability from one area to another.

Lateral changes that prevent continued migration of hydrocarbons in a potential reservoir lithology. They are directly related to their environment of deposition. One type of trap is called the lenticular trap.

1.6.5.   Lenticular Traps

A porous area surrounded by non-porous area. Pinch out or lateral graded trap-differential deposition when environmental deposition changes up-dip.

1.6.6.   Angular Unconformity

Older strata dips at an angle to newer formations. In most cases the older petroleum bearing rocks are subjected to the forces of younger non-porous formation.

2.  Geological Time

2.1.              The Principle of Uniformity

This states that the processes operating today affecting the earth have been operating unchanged at the same rates throughout the earth’s history.

2.2.              Relative Time

The occurrence of a geological event relative to each other. This helps in the dating of geological occurrences. It helps with the identification of a geological continuum.

  • Superposition – the study of layered rocks. It lands lends that older rocks are deposited first and are at the bottom of the sequence. The younger rocks are deposited at the top.
  • Succession – the deposition of flora and fauna, refers to the deposition of sedimentary material. The fossils of these organisms will be found in the rock formations. Thus the presence, absence or change within a sequence could indicate a possible correlation or rock formations.
  • Inclusions – Inclusion of one rock type into surrounding rock is invariably younger rock than the rocks that it intrudes. Good for determining relative age.
  • Cross cutting – Pre-existing rocks that can be affected by later folding and faulting of the rock masses.
  • Physiographic – the development of the earth’s surface referring to the landscape and landforms.

3.  Sedimentary Environments

3.1.              Facies

A body of rock with specific characteristics. Defined on the basis of colour, bedding, composition, texture, fossils and sedimentary structures.

3.1.1.   Facies Sequences

A sequence of facies as the pass from one to another. The sequence can have an abrupt or erosive boundary or a hiatus. In this case there is a coarsening upward sequence or a fining upward sequence. This is important because grain size is an indication of the hydraulic power with which the grain is deposited. An upward coarsening of the particle indicates increased flow an upward fining would indicate low flow.

3.2.              Continental Environments

3.2.1.   Alluvial Fans

Localized and their shape approximates a cone. They develop in regions of high relief where there is an abundant supply of sediment. They develop in humid environments but are best known in and regions where erosional process are NOT as active.

The size of the basin receiving the sediment determines the size of the fan. Most fans have an upward concave profile.

Lithology – matrix and clast supported conglomerates texturally and compositionally immature argillaceous sandstones. Often red beds.

Structures – generally massive some ross-bedding and imbrication in the conglomerates.

3.2.2.   Braided Stream Environments

Areas where river flow diverges and merges. The pattern is as a result of bars that develop and split the river flow.

Longitudinal bars – channel shaped

Curved bars – usually extension and modification to the flanks of longitudinal bars.

Transverse bars – common in sandy low sinuosity streams.

Crevase splay deposits are thin sandstones within floodplain claystones and siltstones deposited by meandering rivers that have breached its banks.

Meandering rivers tend to stay in narrow meander belts building facies of point bar and flood plain. This produces a ribbon like geometry of sand stone bodies. The belts can change course by a process known as avulsion.

Lithology – coarse sand and conglomerates often red with NO organic material. Sandstones Arkosis and/or lithc. Thin intraformational conglomerates are common.

Structures – tabular and trough cross-stratification caused by migration of bars sand waves and dunes. Imbrication is common. Minor cross lamination caused by ripple migration in a abandoned channel or in near full channel.

3.2.3.   Meandering River Deposits

More developed distribution of channel processes and greater distinction between channel and over bank deposits than braided streams. They occupy only a small portion of the flood plains at a time. Point bars have approximately horizontal surface at about the same level as the flood plains. Scroll bars are ridges of sand that develop some distance down the point bar surface and are elongated approximately parallel to the contours of the surface.

A fining upward sequence and are cross bedded with upward reduction of set size.

Lithology – medium to fine sandstones claystones and siltstones with a roughly 505/50 ratio of sandstones to mudrocks. Contains conglomerates and generally arkosic, caliche may be present.

Structures – channel surfaces, flood plain deposits may have thin crevasse.

3.3.              Eolian Facies

Sediments that are reworked and laid down through the action of wind currents. Ancient eolian sediments are composed primarily of sand. Usually results in the deposition of sand that is lean and relatively homogenous in size.

Lithology – fine to coarse sandstones well sorted well rounded. Generally quartz areite. Sharp differences in grain size between laminae. May contain minor claystone deposits.

Structures – dominated by large scale cross-bedding. May show mud flakes or adhesion ripples. Often have intraformational unconformities.

3.4.              Barrier Islands & Near Shore Environments

Usually long narrow sand bodies that occur within deltas along deltas and in oceanic an lacustrine environments with no connection to deltas. More likely to form when there is a steady supply of sand to the coast the depositional basin has a limited tidal range and the coastal environmental is stable with a low gradient. They are formed by wave processes.

Backshore – above mean high tide with low angle land and diping laminae maybe eolian dune at top.

Foreshore – between high and low tide swash zone. Low angle seaward dipping laminae. Heavy mineral concentrations.

Shoreface – between low water mark and fair weather wave base. Structurally varied with low angle seaward dipping lamination dune crossbedding. Abundant bio-turbidation.

Upper Offshore – below fair weather base but above storm weather base. Interbedded thin sandstones and claystones may be present. Maybe intense bio-turbidation.

3.5.              Deltaic Environments

Distinct extensions from the shoreline where rivers enter the marine or freshwater depositional basins and supply sediments faster than the basin can redistribute and process them. The general morphology is dependent on the type of sediment, rate of deposition, hydraulic gradient, the energy and the flow of the basins currents.

3.5.1.   Delta Models

  1. Highly constructive deltas formed by fluvial processes
    1. Lobate
    2. Birdsfoot
  2. Highly destructive deltas dominated by basinal processes
    1. Wave dominated
    2. Tide dominated

3.5.2.   Fluvial Dominated Deltas

Unidirectional flow patterns. Highly sinuous patterns are common however braided patterns can be developed. They resemble alluvial channel deposits with erosive basal sequences. Overall fining upward.

They tend to be defined by flood generated events:

  1. Overbank flooding
  2. Crevasse splays. Sediments deposited over a small area.

3.5.3.   Tide Dominated Deltas

Prominent in medium to high tidal ranges. During high sea level marine waters can be trapped in the delta plain. As the tide drops the currents will predominate as the waves escape.

3.5.4.   Wave Dominated Deltas

Distribute the sediment o the delta front. This results in a regular beach shoreline, with a minor protrusion at the month and a steep delta front.       Growth Faults in Alluvially Dominated Deltas

Results from uneven compaction and found in deltas with high shale to sand ratio. The over pressured clays are abnormally high in porosity and relatively plastic when compared to adjacent sediments.

3.6.              Submarine Fans and Turbidites

Turbidity currents deposits deposit sediments these currents are rolling turbulent masses of flow that have dispersed sediments in them. They remain in suspension because of the hydraulic energy in the flow. They are usually deposited on the upper surface of continental slopes, eventually the deposited material will lose its cohesive strength and the surrounding waters will churn the sediments producing the turbide flow. Heavier sediments settle first, followed by smaller grains and then clays.

Distal Fan Turbidites – very thin with no scour marks.

Proximal Turbidites – can have abundant thick lenticular channel fill sand inter-bedded with more classic turbidites.

Lithology – turbidites are transported in chaotic flow regimes

3.6.1.   Exploration Considerations

Turbidite deposits are of recent interest in exploration because they are rich in hydrocarbons. Oil migration will be toward the fan apex. An anticline may develop seaward and could entrap large oil reservoirs.

Environment Trap Type
Eolian Pinchout
Continental Fluvial Channel
Strike Valley
Barrier Bar Shoestring
Coastal Channel
Coastal Delta Crevase Splay & Mouth Bar
Growth Fault related
Deep Marine Submarine Fan pinchout
Paleotopographical Closure
Submarine Channel

4.  Bibliography

Baker Hughes. 1999. Petroleum Geology. Houston TX.

Dikkers, A.J. 1985. Geology in Petroelum Production. Nuew York: Elsevier.

Halliburton. 2001. Basic Petroleum Geology and Log Analysis.

Marshak, Stephen. 2012. Essentials of Geology. Uper Saddle River New Jersey: Prentice Hall.

Payne, Michael L., and Lance D. Underwood. n.d. Halliburton Petroleum Well Construction. John Wiley & Sons.

University of Maryland. 2016. GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:. February 1. Accessed October 11, 2016.


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