Geology and Geophysics

Unlocking the Earth's hydrocarbon potential

Finding oil and gas is no easy task. Geologists and geophysicists are in charge of working out where oil and gas might be trapped within the Earth's crust, and how we can best tap into these reservoirs. This page explains how we decide where we will drill wells, the tools we use to assess what we find, and the models that help our scientists learn more about the rock and the hydrocarbons trapped within it.

01 - SEISMIC ACQUISITION

Seismic data acquisition is the most commonly used geophysical exploration technique and can be shot both on and offshore. Seismic waves are generated and reverberate through the rock strata and are reflected back. This enables geophysicists to estimate the distribution of rock units beneath the Earth’s surface over a large area.

DID YOU KNOW?
Seismic surveys are large, logistical operations that demand effective HSSE management. During a recent survey in the Oman desert, BG Group employed 240 people for a year and laid 1.9 million sound receivers into the ground.

02 - SEISMIC INTERPRETATION

These images, known as seismic sections of the subsurface, are pictures of the rock’s properties, governed by the speed at which the seismic energy travels through the rock and the rock’s density. The sections are structurally interpreted for features such as rock strata, folds and faults, but also contain data which through quantitative interpretation can assist the identification of rock type and fluid content.

DID YOU KNOW?
The data looks small in this picture, but scale can be misleading. In reality, the distance from a red to a black line is equivalent to half of the height of Big Ben, 96 metres.

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03 - EXPLORATION WELLS

Exploration wells are drilled in order to gather evidence to determine whether our Petroleum System is working. We need to find a Hydrocarbon Source, a Reservoir, and a Trap, collectively known as a Play. We also need to find a structure that is large enough to be economically viable.

BG Group has drilled wells in a variety of situations from mountainous terrain (e.g. Bolivia) to deep water (e.g. Brazil). Wells can be drilled vertically or can be deviated: horizontal wells can be drilled for several kilometres. Drilling is a complex, expensive and potentially hazardous process. This is why Safety is our top priority at BG Group and is our first consideration in all drilling operations.

DID YOU KNOW?
Wells are drilled to target depths of up to 6km and aim to hit a target area of 50m x 50m. Have a look at the model drill bit in the display cabinet. It normally weighs 30 kilos and costs $50,000-100,000. Twelve bits are used on average for a single well. This represents less than 1% of the overall well cost.

04 - WELL DATA

Wireline logging consists of lowering a series of detectors on a wire into the borehole after a section of the well is drilled. The resistivity, density, sonic velocity and radioactivity of the rock formations is measured.

Data obtained from the detector provides diagnostic information which is used by geologists to interpret the structure, rock type and reservoir characteristics of layers containing oil or gas.

DID YOU KNOW?
The wireline logging equipment is exposed to pressures that can exceed 15,000 pounds per square inch and can reach temperatures of 350° Fahrenheit/ 177° Celsius. Equipment is extremely expensive and there is always a risk of losing it in the hole whilst conducting the logging operation.

05 - OUTCROPS AND CORES

Geoscientists use analogues exposed on the Earth’s surface today as well as ancient examples that are preserved in the rock record. The aim is to find similarities between what we can see today and the data gathered from the subsurface. The images show us some examples of river channels in the present day and an ancient channel system preserved in a cutting. The core shown in the display shows evidence of a channel base in a North Sea well. We use analogues to estimate some of the uncertainties and risks in defining our field.

Ancient channel system observed in a cutting, Ebro Basin, Spain

Small channels feed into the large mouth of the Ganges river, India

Meandering rivers similar to those observed in subsurface Egypt

	Ancient channel system observed in a cutting, Ebro Basin, Spain

	Small channels feed into the large mouth of the Ganges river, India

	Meandering rivers similar to those observed in subsurface Egypt

DID YOU KNOW? The best hydrocarbon reservoirs are made from ancient beaches, coral reef systems, deserts, and rivers. Hydrocarbon sources would be found in the present day in places such as the Baltic Sea or the Black Sea.

06 - RESERVOIR MODEL

These 3D models incorporate seismic, well and analogue data to replicate the reservoir characteristics deep beneath the surface, and predict its dynamic behaviour. Our experts use these models to calculate the amount of reserves in place, to determine how freely the reservoir will flow and to pinpoint the best place to drill wells to tap into the hydrocarbons and maximise recovery.

DID YOU KNOW?
Although wells tend to be relatively small, no bigger than the diameter of a lamp post at the bottom of the hole, reservoirs can stretch for many miles. BG Group’s recent Tupi discovery is almost as big as Greater London.