Category Archives: Ingenieria Geologica
MODELS OF DINAMIC GEOLOGICAL SYSTEMS
As a rule, geologic systems, with subsequent technologic impact change either in ‘‘geologic’’ or ‘‘technologic’’ time scale. Thus, in order to develop adequate dynamic models of geologic and technologic processes, it is necessary to introduce time factor. The time factor is of a special importance for the problems of any forecasting. Such problems indeed call for the creation and application of the mathematical models. The successful forecast may depend on the historical evaluation of he geologic system under study.
The necessity to take geologic time into account meets with significant difficulties. One of the reasons for this difficulty is the use of absolute and relative geologic time scales. The difference between them is substantial: the absolute time scale has the beginning common for the entire Earth, which is not an attribute of the relative time scale based on paleontology and stratigraphy. Another reason is the lack of repro- ducibility of the geologic time in physical and chemical experiments. Two methods in constructing the dynamic geologic models may be offered: analytical and statistical. The better approach is modeling such systems is the combination of mathematical analysis (i.e., differential equations) with the statistical-probabilistic assignment of numerical values for the parameters, causing the change in dynamic geologic systems.
This approach allows the deterministic description of main features of the dynamics of the geologic systems. At the same time, it allows to account for the statistical-probabilistic nature of various geologic parameters, which cause
the evolution of the systems. The implementation of analytical solution is accomplished using the statistical sampling technique or the so-called Monte Carlo method.
Two important issues must be addressed before constructing analytical models:
1. The key properties of the system under study, as well as those of the surrounding rocks, should be defined. These properties should be described by strictly defined quantitative constraints.
2. The limitations assumed in describing these properties should be clearly delineated and should reflect the substance of a particular geologic system.
It is natural to choose as the main parameters those properties of the system and of the surrounding rocks, which would stimulate or restrain the course of the geologic processes. In the following discussion, the writers use as synonyms the properties of the geologic system and their respective parameters. They may have a dual nature, i.e., they may be either deterministic or stochastic, depending on the formalization approach at each stage of simulation of a geologic system.
Two significant assumptions ought to be made while developing the differential equations of geologic processes.
1. The rate of change of the geologic system, or the speed of the geologic process, is proportional to the state of the system.
2. Influence of various natural factors is proportional to the product of the number (or quantitative estimates) of the events accelerating the process by the number (or quantitative estimates) of the events retarding the process.
Statistical approach, based on the empirical data, is simpler than the analytical one and is justified from the viewpoint of lithosphere evolution. It is based on the inference of interconnections through generalization, analysis, and comparison of the structural–functional features of geologic systems at certain discrete moments of the geologic time. Approximation of the discrete (discontinuous) data by a continuous function allows to obtain an empirical equation for a parameter (or a set of parameters) of the geologic system under study as a function of time.
The relationship between porosity of shales and depth of burial was studied by numerous investigators, this is due to the fact that porosity of argillaceous sediments is a complex function of numerous natural factors, often superimposed on each other. These factors include:
Effective stress (total overburden stress minus the pore pressure);
Speed of sediment deposition;
Thickness of sedimentary formations;
Shape and sorting of grains;
Amount and type of cementing material;
Chemistry of interstitial fluids.
This multitude of variables complicates the quantitative evaluation of the influence of individual factors on the porosity of argillaceous sediments.
ORIGINS OF PETROLEUM. During certain geologic ages, when the climate was suitable, petroleum began as organic material derived from plants and animals which grew in abundance. As these organisms went through their cycles of growing and dying, buried organic material slowly decayed and became our present day fossil fuels: oil, gas, coal and bitumen. Oil, gas and bitumen were dispersed in the sediments (usually clay-rich shales). Over millions of years, these organic-laden shales expelled their oil and gas under tremendous pressures from the overburden.
The oil and gas migrated into permeable strata below or above them, then migrated further into traps that we now call reservoirs. It’s interesting to note that the word “petroleum” is derived from the Latin words for “rock” (petra) and “oil” (oleum), indicating that its origins ie within the rocks that make up the earth’s crust. These ancient petroleum hydrocarbons are complex mixtures and exist in a range of physical forms — gas mixtures, oils ranging from thin to viscous, semi-solids and solids. Gases may be found alone or mixed with the oils. Liquids (oils) range in color from clear to black. The semi-solid hydrocarbonsare sticky and black (tars). The solid forms are usually mined as coal, tar sand or natural asphalt such as gilsonite. As the name “hydrocarbon” implies, petroleum is comprised of carbon atoms and hydrogen atoms bonded together; the carbon has four bonds and the hydrogen has one. The simplest hydrocarbon is methane gas (CH4). The more complex hydrocarbons have intricate structures, consisting of multiple carbon-hydrogen rings with carbon-hydrogen side chains. There are often traces of sulfur, nitrogen and other elements in the structure of the heavier hydrocarbons.
THE MIGRATION AND TRAPPING OF PETROLEUM
Oil is seldom found in commercial amounts in the source rock where it was formed. Rather, it will be found nearby, in reservoir rock. These are normally “sedimentary” rocks — layered rock bodies formed in ancient, shallow seas by silt and sand from rivers. Sandstone is the most common of the sedimentary rock types. Between the sand grains that make up a sandstone rock body there is space originally filled with seawater. When pores are interconnected, the rock is permeable and fluids can flow by gravity or pressure through the rock body. The seawater that once filled the pore space is partially displaced by oil and gas that was squeezed from the source rock into the sandstone. Some water remains in the pore space, coating the sand grains. This is called the reservoir’s connate water.
Oil and gas can migrate through the pores as long as enough gravity or pressure forces exist to move it or until the flow path is blocked. A blockage is referred to as a trap. Carbonate rock, limestones (calcium carbonate)
and dolomites (calcium magnesium carbonate) are sedimentary rocks and are some of the most common petroleum reservoirs. Carbonate reservoirs were formed from ancient coral reefs and algae mounds that grew in ancient, shallow seas.
Organic-rich source rocks were also in proximity to supply oil and gas to these reservoir rocks. Most limestone strata do not have a matrix that makes them permeable enough for oil and gas to migrate through them. However, many limestone reservoirs contain fracture systems and/or interconnecting vugs (cavities formed when acidic water dissolved some of the carbonate). These fractures and vugs, created after deposition, provide the porosity and permeability essential for oil to migrate and be trapped. Another carbonate rock, dolomite, exhibits matrix permeability that allows fluid migration and entrapment. Dolomites also can have fracture and vugular porosity making dolomite structures attractive candidates for oil deposits.
A significant portion of oil and gas production is associated with salt domes which are predominately classified as piercement-type salt intrusions and often mushroom shaped. Piercement-type domes were formed by the plastic movement of salt rising upward through more dens sediments by buoyant forces resulting from the difference in density.
Major oil and gas reservoirs have been found in recent years beneath horizontal salt beds. Until recently, it was a mystery what was b
eneath these extruded salt layers called salt sills, salt sheets and salt lenses. They could not be explored economically by drilling, and seismic interpretation through plastic salt was unreliable. Now, “sub-salt” formations can be evaluated through modern three-dimensional seismic analysis to identify potential reservoirs. Once likely formations are located, wells are drilled through the salt layer to determine if oil and gas deposits exist.
Structural traps result from a local deformation such as folding and/or faulting of the rock layers. Examples of structural barriers are anticline traps, fault traps and traps associated with salt domes (see Figures 1a, 1b and 2c). Stratigraphic traps are formed by geological processes other than structural deformation and relate to variations in rock properties (lithology).
La información sísmica es uno de los pasos más importantes en la exploración. Esta permite conocer con mayor exactitud la presencia de trampas en el subsuelo.
La sísmica consiste en crear ondas sonoras artificiales mediante el accionar de pequeñas cantidades de un material especial llamado sismigel, que se ubica en pequeños pozos de 8 centímetros de diámetro y entre 5 y 15 metros de profundidad, buscando que las ondas se propaguen hacia el subsuelo y evitando daños en el medio ambiente. A medida que las ondas se propagan hacia el interior de las capas de la tierra, se producen pequeños ecos que son percibidos solamente por pequeños aparatos de alta sensibilidad llamados geófonos, los cuales se colocan sobre la superficie del terreno.
Los geófonos van unidos entre sí por unos cables que transmiten los ecos percibidos hacia una unidad central de registro
en las etapas iniciales de planeacion de la exploracion es una nueva area, se usan ampliamente exploraciones sismicas superficiales para delinear las trampas estructurales o estratigraficas. Mejoras recientes en las tecnicas de filtrado y procesamiento digitales ha conducido a resultados de alta calidad en condiciones favorables.
Al inciar la perforacion, existen oportunidades para mejorar esta situacion a travez del uso de registros de pozo. Despues de corregir y calibrar contra disparos de verificacion, los registros sonicos y de densidad pueden utilizarse para generar sismografos sinteticos, los cuales, son extremadamente valiosos para verificar las reflexiones en una seccion sismica y relacionar las carqacteristicas sismicas con estructuras geologicas.
Una ampliacion geofisica reciente de los registros de servicio de cable implica la preparacion de un Perfil Sismico Vertical (VSM). En esta tecnica, un cañon de aire u otra fuente sismica en la superficie genera la señal de entrada que un geofono de pozo detecta. Ya que la energia sonora viaja solo una vez a traves de las capas superficiales , el perfil resusltante tiene una resolucion mucho mejor que la sismica superficial alrededor del agujero y, en casos favorables, pueden identificar reflectores ubicados muy por debajo de la profundidad maxima del pozo.
EQUIPO SISMICO PARA POZO:
Estos conssiten en una herramienta de pozo con geofonos, el sistema de registro superficial CSU, el equipo de fuente desplazada y un cañon de aire u otra fuente- normalmente en plataformas marinas el uso del cañon de aire es aceptable-
Ademas tambien se puede usar un arreglo de cañones sincronizados. para tener una penetracion profunda.
Todos los datos se registran en forma digital sobre cinta magnetica con el sistema CSU. Las ondas sismicas tambien pueden presentarse en forma analogica sobre una pelicula y pueden agrupar varios disparos realizados en el mismo nivel. Las Herramientas actuales empleadas son las herramientas sismicas de Pozo WST y la herramienta de Adquisicion Sismica SAT. La primera tiene 4 geofonos uniaxialmente mientras que la segunda solo tiene 3 geofonos.
ADQUISICION DIGITAL DE DISPAROS DE VERIFICACION:
En cada profundidad se mide la velocidad de intervalo de las formaciones entre fuente y el geofono de agujero. El hidrofono supervisda la identificacion y la sincxronizacion de la señal de la fuente y el geofono registra las llegadas directas y reflejadas.. El tiempo de transito se mide desde el momento inicial del registro del hidrofono (surface) hasta el tiempo inicial del registro del geofono (subsurface).
Si el agujero esta desviado o existe un desplazamiento considerable de la fuente, los tiempos de transito obtenidos deben convertirse en tiempos de transito de profundidad vertical real TVD. La correcion en un plano de referencia sismico SRD tambien es necesaria si la fuente esta por arriba o por debajo del plano de referencia.
- Pozo donde se ubica el sismigel, el cual al ser accionado genera ondas sonoras artificiales.
- Ondas sonoras que viajan hacia el interior de la tierra.
- Ondas transmitidas (ecos) hacia los geofonos en superficie.
- Geofonos colocados en la superficie que captan las ondas sonoras.
- Trabajo de topografica para conocer la posicion de los geofonos y los pozos.
- Estacion receptora movil que se encarga de recopilar los datos suministrados por los geofonos para ser procesados.
- Equipos de computo especializados para procesar e interpretar la informacion obtenida.
A PREVIOUS STUDY OF HUALLAGA BASIN. The Huallaga basin was not seriously explored until the early 1990’s when Mobil signed four concessions totaling 36,000 km2, covering almost the entire basin. Mobil acquired the first seismic shot in the basin and through three seismic campaigns, recorded a total 1600 km of data. After drilling the Ponasillo 1 X in 1992, which was plugged and abandoned as a dry hole, Mobil relinquished their acreage in 1993. The Huallaga Basin has largely remained dormant from an exploration viewpoint since that time.
The fold and thrust belt separating the Huallaga from the Marañon Basin was first seriously explored in 1996 with the signing of Block 72 by Occidental Peruana. In the process of evaluating the Block, Occidental reprocessed 615 km of seismic data in the Huallaga area and acquired 148 km more. Occidental relinquished the area in 1999 without drilling a well.
In 2000, Advantage Resources signed Block 87, which has a configuration similar to that of the old Occidental Block. As of this writing, Advantage with their partners Burlington Resources, had completed extensive geological fieldwork in the Huallaga Area reprocessed 409 km of Mobil and Occidental seismic data and acquired an additional 201 km of seismic data. This recent seismic data acquisition was not interpreted as part of this study.
The southwestern Marañon and northwestern Ucayali Basins bordering the Huallaga fold and thrust belt, have been the recipients of considerable more activity in years past. The first significant exploration in southwestern Marañon was done by Texaco in the 1950’s whose work culminated with the drilling of the Yurimaguas 1X well. Exploration returned to the area in the late 1970’s with Deminex in Block 12. In the process of their evaluation, Deminex acquired 2890 km of seismic data and drilled two wells, Loreto 1X, and Shanusi 1X, the later of which had significant gas shows in the Pucará Formation. Since the departure of Deminex in 1970’s, only minimal work has been done in the area. Another activity was a one line seismic acquisition by Coastal Petroleum in 1998 that was shot to confirm the northwest closure of the Yurimaguas structure. This acquisition was part of Coastal’s much larger exploration activity that was focused primarily on the area surrounding the Contaya Arch and the northern Ucayali Basin when they controlled Blocks 73 and 74 between 1994 and 1999. During this time Coastal reprocessed 2890 km of seismic data, acquired 380 km in 1995 and 261 km in 1998. Before relinquishing their blocks, Coastal drilled three wells, Orellana 1X, Santa Catalina 1X and Insaya 1X. All three wells were plugged and abandoned.
The Huallaga Basin for all intensive purposes is considered a tectonic basin formed by Andean compressional deformation during the Late Tertiary prior to which it represented the western reaches of the greater Marañon basin. The present day Basin is 400
km long from north to south with a maximum width of 100 km. A large linear regional structurally complex uplift separates the Jurassic-Tertiary sequence of the Huallaga basin from the equivalent section in the Marañon basin. This frontal belt is a complex product of inversion, thrust and/or salt tectonics on which rocks as old as Triassic are exposed. The youngest rocks flanking the uplift are Neogene in age. Similar to the western Marañon basin, the Lower Cretaceous beds of the Huallaga rest directly upon a pre-Cretaceous Mesozoic section that includes Jurassic–Triassic carbonates and evaporites, which in turn overlies an unknown but presumably Paleozoic stratified sedimentary section.
The geological evolution of the study area is controlled by two regional tectonic systems recognized in the sub-Andean basins of Peru. The first, the pre-Andean System, encompasses three cycles of Ordovician, Devonian and Permo-Carboniferous ages overlying the Precambrian basement of the Guyana and Brazilian Shields. The second, the Andean System, was initiated with the beginning of subduction along the western margin of Peru. It encompasses several mega-stratigraphic sequences and numerous minor sedimentary cycles, ranging from late Permian to the Present.