Geology & Geophysics

Cenozoic is the most recent geological era that started around 70 +/- 2 million years ago. Cenozoic era is subdivided into 2 different periods. These subdivided periods in reverse chronological order from oldest to latest are Tertiary and Quaternary.

Clastics, also known as Terrigenous Sediments are minerals formed from the fragmentation of silicate rocks. These are derived from land and are deposited following a sedimentary cycle. Clastics are further categorized as Clay, Silt, Sand and Rudite based on the particle size.

Clay is a clastic with very fine particles having diameters less than 0.004 mm. It's chemical name is a 'hydrous platy aluminosilicate' and is the finest grade of clastic minerals. Clays are further sub-divided in five different groups based on chemical and physical characteristics as Kaolinite, Illite, Smectite, chlorite and Glauconite.

Cretaceous is the period of Mesozoic geological era. In geological time scale, Cretaceous period started around 135 +/- 5 million years ago and lasted for about 65 million years.

Devonian is a period within Palezoic geological era. In geological time scale, Devonian period started around 350 +/- 50 million years ago and lasted for about 50 million years. Devonian period is considered significant since the life is thought to have spread from ocean to land during this period.

Evaporites are sediments which include salt minerals like Anhydrite, Gypsum, Halite etc. Evaporites however are not deposited through sedimentary cycle like other sedimentary rocks but through the precipitation sequence from brine. Evaporites are usually found in thick beds and form good cap rock for oil and gas entrapment.

Fracture Closure Stress (FCS) is based on the concept that the wellbore is held together due to the built-in stresses within the wellbore. The overburden pressure and pore pressure laterally translate into creating these stresses. FCS describes the mechanism of how the fracture in a wellbore initiates and propagates. Whenever the pressure inside the wellbore exceeds the stress holding the wellbore closed which is the ‘Fracture Closure Stress’, the fracture will initiate and propagate.

The pressure at which the rock breaks down is known as Fracture Pressure. Fracture pressure is higher than pore pressure and less than overburden pressure. Pore pressure and Fracture pressures are very important for designing a well. The specific gravity of mud is computed in such a manner that the hydrostatic pressure created by the column of mud is more than pore pressure but less than Fracture Pressure at any point in the well. In a deep water environment, the window between pore pressure and fracture pressure becomes narrow.

Casing points are carefully selected to ensure that the open hole is cased off before the Hydrostatic Pressure of the mud exceeds the estimated fracture pressure. Since Fracture Pressure while planning a well is an estimated value based on offset well data, ‘Leak Off Test (LOT)’ or ‘Formation Integrity Test (FIT)’ is carried out below critical casing shoes to calibrate Fracture Pressure at that point in the well. Actual values help extrapolation for the remaining part of the well and provide baseline value for future wells in the area as well.

Stress Caging - An effective wellbore strengthening approach

Geostatic Pressure is also known as Overburden pressure, Lithostatic Pressure, Confining Pressure, Vertical Stress, Total Stress. It’s defined as the pressure exerted by weight of rock and fluid in the rock on the formation layer at the point of interest. The equivalent density of the combined weight is referred to as the bulk density (ρb). Geostatic Pressure is normally estimated as average 1 psi/ft but varies from basin to basin depending on matrix density of rocks.

Jurassic is the period of Mesozoic geological era. In geological time scale, Jurassic period started around 165 +/- 10 million years ago and lasted for about 30 million years.

Lithostatic Pressure is also known as Geostatic Pressure, Overburden pressure, Confining Pressure, Vertical Stress, Total Stress. It’s defined as the pressure exerted by weight of rock and fluid in the rock on the formation layer at the point of interest. The equivalent density of the combined weight is referred to as the bulk density (ρb). Lithostatic Pressure is normally estimated as average 1 psi/ft but varies from basin to basin depending on matrix density of rocks.

A log is defined as a systematic record of data. In the oil field, logging refers to the process of recording formation information downhole for evaluating properties and reservoir characteristics. A well log is a plot of different formation properties versus depth.

Interpretation of well logs can help identify lithologies, porosity, permeability, etc and provide valuable information in identifying a pay zone. Various different types of tools are run in the well on either wireline or drill pipe to measure different formation properties. Logging tools use electrical, electronic, sonic, and radioactive materials to be able to measure specific rock properties.

Mesozoic is a geological era that started around 200 +/- 20 million years ago. Paleozoic era is subdivided into 3 different periods. These subdivided periods in reverse chronological order from oldest to latest are Triassic, Jurassic and Cretaceous.

Overburden pressure is also known as Geostatic Pressure, Lithostatic Pressure, Confining Pressure, Vertical Stress, Total Stress. It’s defined as the pressure exerted by weight of rock and fluid in the rock on the formation layer at the point of interest. The equivalent density of the combined weight is referred to as the bulk density (ρb). Overburden pressure is normally estimated as average 1 psi/ft but varies from basin to basin depending on matrix density of rocks.

Paleozoic is a geological era that started around 550 +/- 50 million years ago. Paleozoic era is subdivided into 7 different periods. These subdivided periods in reverse chronological order from oldest to latest are Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian and Permian

Pore pressure is the pressure of the fluid inside the pore spaces of the rock. It is also known as Formation pressure. If the pore pressure is higher than 'Normal Hydrostatic Pressure', it is called Abnormal and if it is lower than the normal hydrostatic pressure at a particular depth, it is called Subnormal pressure. Normal hydrostatic pressure is the pressure exerted by a vertical column of water with salinity normal for the specific geographic area.

The specific gravity of drilling fluid is planned such that the hydrostatic pressure created by the column of mud is more than pore pressure at any point in the well. This difference is called over-balance. Every company has safety policies to maintain pre-defined minimum over-balance depending on the type of well and reservoir fluid expected in a well.

Pore throats are openings between grains of formation rocks through which the formation fluid or fine particles may travel.

Types of Loses

Causes, Effects, and Control of Seepage Loss

Precambrian is the oldest geological era. The life on earth started in ocean sometime in Precambrian era and spread to land sometime in Devonian period.

Unlike Terrigenous Sediments (Clastic), which are derived from land, Pyroclastic Sediments are derived from volcanic eruptions. They may closely resemble the Terrigenous Sediments, for they may have gone through similar weathering and abrasion cycle after falling above the existing sediments as a result of eruptions.

Quaternary is the most recent geological period and falls under Cenozoic era. Quaternary period stated around 0.5 million years ago.

Reservoir is a rock formation that is capable of containing oil, gas or water. To be able to produce and be commercially viable, the reservoir should be thick and vast. It should have required porosity and permeability to be able to accumulate and flow the reservoir fluid. Sandstone and Carbonates are most common conventional reservoir rocks.

Reservoir geochemistry is the study of the chemical composition of reservoir rocks and fluid in the reservoir. The reservoir fluid can be either water, oil, or gas. Their chemical analysis and study of their interactions with the reservoir rock and the surrounding formation provide valuable information. Reservoir geochemistry helps understand reservoir fluid's origin, migration, and accumulation. It also aids in evaluating reservoir quality, compartmentalization, and potential productivity of a reservoir.

Rudites are sediments with particle size more than 2 mm. This is a coarse rock segment and could be either round or angular shaped. The round particles which have undergone abrasion process are called Conglomerates and the angular ones are called Breccias.

Sands are clastic sediments with particle diameter ranging between 0.04 and 2 mm. Sand sediments usually have wide lateral extent, high porosity and permeability. Sand stones are major aquifers and hydrocarbon reservoirs in different fields.

Silts are clastic sediments of intermediate size between clay and sand. Diameter of silt particles range between 0.004 and 0.06 mm. Most clays and clay stone contain about 30 % or more silt. Silt could be very abrasive causing significant bit wear.

Stratigraphic trap is said to exist when a non-porous formation seals off the top edge of a reservoir. The fluid in the reservoir rock migrates up as far as it finds good porosity and permeability and then stops at the non-porous formation. This phenomenon results in trapping the sub-surface accumulation.

Structural traps are result of faulting or folding. It occurs when the migration of reservoir fluid is halted as a result of a non-porous formation blocks the flow due to shift of formations.

Terrigenous Sediments, also known as Clastics are minerals formed from the fragmentation of silicate rocks. These are derived from land and are deposited following a sedimentary cycle. Clastics are categorized as Clay, Silt, Sand and Rudite based on the particle size.

Tertiary is the period of Cenozoic geological era. In geological time scale, Tertiary period started around 70 million years ago and lasted till very recent time. Tertiary period had six different subdivisions called epochs. These epochs are Paleocene, Eocene, Oligocene, Miocene, Pliocene and Pleistocene in reverse chronological order from oldest to latest.

Once hydrocarbons are formed, they migrate upwards from the source rock. If this migration continues unabated, the hydrocarbon would eventually escape on the surface. Sub-surface accumulation is a result of the reservoir fluid hitting a barrier which doesn't allow it to migrate any further. This barrier is called Trap. It could either be a structural trap or a stratigraphic trap or a combination of two.

Triassic is the period of Mesozoic geological era. In geological time scale, Triassic period started around 200 +/- 20 million years ago and lasted for about 35 million years.