Petroleum Exploration Part 1.2 Petroleum Exploration

Objectives After reading the chapter and reviewing the materials presented the students will be able to: Understand the evolution of seismic surveys and interpretation Identify the types of well logs and core samples. Identify contour maps and digital computer models.

Introduction Surface and subsurface geological studies drive the discovery of oil and gas. Seismic data, well log data, aerial photographs, satellite images, gravity and magnetic data, and other geological data provide information that help determine where to drill an exploratory well. Specialists examine rock fragments and core samples brought up while drilling the exploratory well and run special tools into the hole to get more information about the formations underground. Examining, correlating, and interpreting this information makes it possible for petroleum explorationists to accurately locate subsurface structures that might contain hydrocarbon accumulations worth exploiting.

Surface Geological Studies Petroleum explorationists study the topography – the natural and man made features on the surface of the land – to derive a conclusion about the character of underground formations and structures largely from what appears at the surface. Remote sensing has largely replaced aerial photography. Remote sensing uses infrared and other means to map an area. Both satellites and airplanes are used to conduct remote sensing.

Landsat For over 30 years NASA Landsat satellites have collected information about all of the earth’s landmasses. Geologists use Landsat data to detect the presence of clays often associated with mineral deposits. Landsat data can be ordered from the U.S. Geological Survey (USGS). Another type of remote sensing uses radar. Imaging radar used in airplanes is called side looking airborne radar (SLAR).

Oil and Gas Seeps Some of the world’s greatest oilfields were discovered because of the presence of oil seeps on the surface. Oil seeps occur either along fractures that pierce the reservoirs or at spots where formations rise up to the surface.

Collecting Data Private companies or geological societies often maintain large collections of drilling and production data, maps, or well logs (data taken from existing wells). Members pay dues for access to the information. Oil and gas regulatory agencies in the United States collect and file all types of data relating to drilling and production. Both public and private organizations have established computer databases that offer access to regional information that is classified by field and reservoir. Most major oil companies subscribe to petroleum databases, and nonsubscribers can request service on a per search basis.

Geophysical Surveys Petroleum geologists are mainly interested in the earth’s magnetism, gravity, and especially seismic vibrations of the earth. Sensitive instruments can measure variations in one of these physical quantities that might be related to conditions under the surface. These conditions could point to promising oil and gas bearing formations.

Magnetic and Electromagnetic Surveys Through magnetic surveys, geophysicists obtain a fairly clear picture of the geological formations. Comparable rocks have similar magnetic fields. A magnetometer is a scientific instrument that detects slight variations in the earth’s magnetic field. These variations indicate how deep magnetized rocks are buried. These surveys are extremely useful in giving the geologist a better idea of where oil bearing rocks may be found.

Seismic Surveys Seismic surveys give geoscientists more precise details on the formations beneath the surface. Information from seismic surveys indicates the type of rock, their relative depth, and whether a trap is present. The advantage of 3D seismic surveys is that an geoscientist can use computers to slice the cube in any direction to provide much more reliable information about the geologic structures. The term 4D seismic refers to repeated 3D surveys through time as the fourth dimension. 4C seismic is used in marine exploration to record both pressure and converted shear waves.

Seismic Surveys Engineers have developed mechanical impactors and vibrators to create seismic waves on the earth’s surface that penetrate down into the rock layers. The reflections are picked up and changed into electrical impulses by geophones. These recorded impulses are sent to a computer for analysis and printed as a seismic reflection profile. Geologists analyze the profile to determine subsurface structures. Geologists may also use geophones in the borehole of an existing well. In this method, technicians run geophones in a well and attach them to the wall of the hole at intervals of 20 to 100 feet. It can gather information about geologic structures in the immediate vicinity of the hole.

Marine Seismic Methods A sound source sends sound waves through the water and formations beneath the seafloor reflect the seismic waves to hydrophones, the marine version of geophones. Commonly hundreds of hydrophones trail behind the ship on steel cables.

Ocean Bottom Cable Systems The OBC (Ocean Bottom Cable) systems acquire full seismic wave data from the ocean floor. Data noise is reduced (cables are not vibrating due to towing, weather, or currents). Today an air gun is the most common sound source used on seismic ships. A satellite system developed by the US government and the oil industry can receive data directly from the ship and beam it back to a receiver on land where it is immediately sent to a computer. The computer translates the data into a map that is sent to the exploration company office. There geoscientists are able to see it and direct the ship’s captain to focus on particularly promising formations.

Reservoir Development Tools If the surface and subsurface information indicates a strong possibility of hydrocarbons, the oil company might drill an exploratory well or wells. Well logs are used to record information about the formation through which a well has been drilled. The driller’s log provides basic information to the geologist. They keep a record of the kinds of rocks and fluids encountered at different depths, along with anything else of interest. A wireline is a metal line that can be run into the hole with a tool attached to the end. A sonde (sophisticated electronic instrument) is lowered in the well bore on a conductor line and measures and records electrical, radioactive, or acoustic properties of formations as it is raised to the surface at a predetermined speed. At the surface the wireline unit has computers that translate these signals into graphs for geologists and engineers to interpret. Correlating the information from the different types wireline logs can give the geologist valuable information about the rock type, porosity, permeability, and the amount of water and petroleum in the formation.

Reservoir Development Tools Because saltwater conducts electricity much better than oil and gas, resistance and induction logs help determine how much water is in the formation, how freely the water moves (permeability) and how saturated the formation is with water rather than hydrocarbons. A gamma ray log is useful in useful in identifying impermeable formations such as shale and clay filled sands. A neutron log shows how much oil, gas, and water might be in the formation, the type of rock and its porosity, and the salt content.

Sample Logs Sample logs are logs of physical samples of the underground rock. The two types of samples are core samples and cutting samples. A core is a slender column of rock that shows the sequence of rocks as they appear within the earth. It provides accurate information about underground formations. The analysis includes the porosity, permeability, composition, fluid content, and geological age of the core sample. This information helps the geologist determine the oil bearing potential of the cored beds. When a regular drill bit drills a hole, it breaks up the rock into pieces called cuttings. The cuttings are suspended in the drilling mud, and carried up to the surface where geologists use them to analyze to analyze the rock being drilled. Cuttings are not as useful as cores, but can provide useful data and are regularly examined during drilling.

Drill Stem Test The drill stem test (DST) is the principal way to test a formation that has just been drilled. A DST tool is run into the hole. Formation fluids flow into a perforated pipe in the tool. A pressure recorder inside the DST tool and another below the perforated pipe charts the pressure. The DST gives accurate data about the formation’s pressure and the composition of fluids the formation contains.

Stratigraphy Stratigraphy is the study of the origin, composition, distribution, and sequence of rock strata. A strat test involves drilling a hole mainly to obtain geological information. A company uses the strat test on an exploratory well to examine the cuttings for hydrocarbons. Stratigraphy correlation is the process of comparing geologic formations. In oil exploration, stratigraphers compare the geology of a known area with unknown formations in nearby locations to predict the location of new reservoirs.

Maps Geologists use base maps showing existing wells lease and property lines, roads, buildings, and other man made surface features to recommend sites for geophysical studies, exploratory drilling, and reservoir development. Petroleum geologists commonly use contour maps that show topography, gravity, and other surveys. One type of contour map is the structural map, which shows the subsurface contours of a particular reservoir or formation. Cross sections reveal anticlinal and fault traps and any horizontal variations in rock.

Data, Software, and Modeling Technology Geoscientists use many types of software to analyze the data needed (seismic and well data), create maps, create models, and decide where to drill. All data is stored in a well file that contains well information such as a well header (name, identifying number, location, etc.) data, directional drilling data, production data, perforation data, and so on. Sophisticated computer software makes it possible to collect, generate, interpret, and transmit large volumes of well data. Reservoir modeling helps predict future production of the formation so drilling decisions can be made. Geographic information systems (GIS) are a dynamic technology that uses a digital map to display layers of data, which can be shared, analyzed, and evaluated.

Summary Surface and subsurface geological studies drive the discovery of oil and gas. Seismic data, well log data, aerial photographs, satellite images, gravity and magnetic data, and other geological data provide information that help determine where to drill an exploratory well. Private companies or geological societies often maintain large collections of drilling and production data, maps, or well logs (data taken from existing wells). Information from seismic surveys indicates the type of rock, their relative depth, and whether a trap is present. Engineers have developed mechanical impactors and vibrators to create seismic waves on the earth’s surface that penetrate down into the rock layers. The reflections are picked up and changed into electrical impulses by geophones. These recorded impulses are sent to a computer for analysis and printed as a seismic reflection profile. Geologists analyze the profile to determine subsurface structures. Well logs are used to record information about the formation through which a well has been drilled. The driller’s log provides basic information to the geologist. They keep a record of the kinds of rocks and fluids encountered at different depths, along with anything else of interest. A wireline is a metal line that can be run into the hole with a tool attached to the end. A sonde (sophisticated electronic instrument) is lowered in the well bore on a conductor line and measures and records electrical, radioactive, or acoustic properties of formations as it is raised to the surface at a predetermined speed. At the surface the wireline unit has computers that translate these signals into graphs for geologists and engineers to interpret. Correlating the information from the different types wireline logs can give the geologist valuable information about the rock type, porosity, permeability, and the amount of water and petroleum in the formation. A core is a slender column of rock that shows the sequence of rocks as they appear within the earth. It provides accurate information about underground formations. The analysis includes the porosity, permeability, composition, fluid content, and geological age of the core sample. This information helps the geologist determine the oil bearing potential of the cored beds. Stratigraphy is the study of the origin, composition, distribution, and sequence of rock strata. Geologists use base maps showing existing wells lease and property lines, roads, buildings, and other man made surface features to recommend sites for geophysical studies, exploratory drilling, and reservoir development. Reservoir modeling helps predict future production of the formation so drilling decisions can be made. Geographic information systems (GIS) are a dynamic technology that uses a digital map to display layers of data, which can be shared, analyzed, and evaluated.

Home Work 1. What information can be gathered from seismic surveys? 2. What are well logs used for? 3. What is a core and what information does it provide? 4. What are geographic information systems(GIS)?