BASIN ANALYSIS

Sedimentary basin analysis is a geologic method by which the history of a sedimentary basin is revealed, by analyzing the sediment fill itself. Aspects of the sediment, namely its composition, primary structures, and internal architecture, can be synthesized into a history of the basin fill. Such a synthesis can reveal how the basin formed, how the sediment fill was transported or precipitated, and reveal sources of the sediment fill. From such syntheses models can be developed to explain broad basin formation mechanisms. Examples of such basinal environments include backarc, forearc, passive margin, epicontinental, and extensional basins.

Sedimentary basin analysis is largely conducted by two types of geologists who have slightly different goals and approaches. The petroleum geologist, whose the ultimate goal is to determine the possible presence and extent of hydrocarbons and hydrocarbon-bearing rocks in a basin, and the academic geologist, who may be concerned with any or all facets of a basin's evolution. Petroleum industry basin analysis is often conducted on subterranean basins through the use of reflection seismology and data from well logging. Academic geologists study subterranean basins as well as those basins which have been exhumed and dissected by subsequent tectonic events. Thus academics sometimes use petroleum industry techniques, but in many cases they are able to study rocks at the surface. Techniques used to study surficial sedimentary rocks include: measuring stratigraphic sections, identifying sedimentary depositional environments and constructing a geologic map.

An important tool in sedimentary basin analysis is sequence stratigraphy, in which various sedimentary sequences are related to pervasive changes in sea level and sediment supply.

Basin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves.

At its most basic, a basin modelling exercise must assess:

1. The burial history of the basin (see back-stripping).
2. The thermal history of the basin (see thermal history modelling).
3. The maturity history of the source rocks.
4. The expulsion, migration and trapping of hydrocarbons.

By doing so, valuable inferences can be made about such matters as hydrocarbon generation and timing, maturity of potential source rocks and migration paths of expelled hydrocarbons.

Sequence stratigraphy

III. Sequence stratigraphy

Examines sedimentary packages over a large area (say, across the entire continental shelf or ancient sedimentary basin) to unravel the entire geologic history that led to their formation. Although this work can be done using data from outcrops and drill holes, a huge amount of this work is done using seismic stratigraphy.

A.      Sequence stratigraphy focuses on the relationships between sequences of conformable layers and the unconformities that bound them. Although lithologic and biostratigraphic information are certainly useful, it is the geometric relationships between unconformities and conformable strata, generally as seen on seismic records, that is the basis for sequence stratigraphy.

1.        It is convenient to divide the entire stratigraphic record at a given place into a number of depositional sequences.

Depositional sequence: a stratigraphic unit composed of a relatively conformable succession of genetically related strata that is bounded at its top and base by unconformities or their correlative conformities. Memorize this definition. It is a mantra. Its meaning will be made clear

below.

Sequence boundary: an unconformity and any correlative conformities that marks the base or top of a depositional sequence.

2.       Note that the layers are numbered 1 through 25. Wherever the numbers are consecutive, the sediments are conformable; wherever there is a gap in numbers between beds, there is an unconformity. The heavy line "A" represents the basal sequence boundary. From left to right, it is defined by an angular unconformity, then a correlative conformity (the boundary between beds 10 and 11 is of the same age as the angular unconformity), and grades out to a paraconformity. Note that on the right side the layering skips from bed 10 of the underlying sequence to anything between beds 12 and bed 16. Beds 11 through 19 suggest a prograding shelf sequence. The upper horizontal beds are likely to be fairly shallow water sediments (inner shelf/beach) whereas the lower horizontal beds are probably outer shelf sediments. The upper sequence boundary is marked by the angular relationship between Bed 19 and Beds 20 through 24. On the left is a paraconformity (gap between 17 and 24 over most of its length) and in the deep part of the basin is a correlative conformity (horizontal boundary between 19 and 20).

B.       Again, the fundamental basis of sequence stratigraphy is the physical relationships of conformable strata and unconformities. It does not depend on rock types, fossils, or inferred depositional processes.

C.       Time significance of boundaries: boundaries between adjacent conformable strata are regarded as essentially synchronous. Ages of sediments above and below unconformities varies widely along strike.

D.      Vocabulary to describe geometric relations among strata and bounding surfaces

All relations between strata and sequence boundaries are either:

1.       Concordant: layers are parallel to a sequence boundary. This boundary may be horizontal, inclined, or uneven (see figure).

2.        Discordant: layers are not parallel to a sequence boundary Discordance is essential to recognizing sequence boundaries. (A paraconformity by itself would never be distinguished from conformable strata in a seismic record!)

3.        Truncation: where strata are terminated by erosion, either regionally or locally, along the upper boundary surface.

4.        Apparent Truncation: where down-dip ends of strata terminate at the upper surface in what was probably a surface of non-deposition. There is no clear evidence for scouring.

5.        Lapout: when strata terminate against a sequence boundary at their original depositional limit. No significant erosion is involved. Lapout occurs at either the upper or lower boundaries. There are 4 terms for different types of lapout (6 through 9 below):

6.       Toplap: lapout at the upper sequence boundary. This results from sediments filling up a basin and being forced to prograde outwards. For example, they could be the foreset beds of a delta or of beach and inner shelf sediments. While there is probably some erosion along a toplap surface, this erosion was associated with the original deposition of the sequence.

7.       Baselap: lapout at the lower sequence boundary. Two types:

8.       Onlap: baselap in which successively younger strata march up an inclined surface, as would happen during a transgression.

9.       Downlap: baselap in which inclined strata terminate downdip against a horizontal or inclined surface, as would happen as delta front strata prograde across a basin. You can see similar relationships in cross-bedding in an outcrop, which is entertaining, but the above vocabulary is reserved for sequence stratigraphy.

E.       Relationship between seismic surfaces and lithology: somewhat indirect. Reflectors indicate some density contrast between layers. Often occurs across an unconformity, of course, but also often occurs when there is a sudden change in relative sea level. This puts suddenly coarser or finer sediments on top of older sediments, thus creating a contrast.

F.        Sequence stratigraphic units: built on geometry of discordant surfaces

1.        Sequence boundary: marked by seaward shift in facies, downward shift in costal onlap, subaerial exposure, ± erosion by stream down-cutting (Boggs Fig. 15.2). (They used to distinguish Type 1 and Type 2 sequence boundaries, but no longer.)

2.       Depositional Systems: 3-dimensional depositional sequences: assemblages of genetically related lithofacies bound between unconformities and their correlative conformities. Depositional systems are made up of one or more systems tracts:

3.       There are 4 kinds of Systems Tracts: (Boggs Fig. 15.3)

a. Highstand Systems Tracts: Progradational sequences that form during the late part of a sea level rise, stillstand, or earliest part of a fall. They lie immediately below the sequence boundary formed by a succeeding sea level fall.

b. Lowstand Systems Tracts: Formed during a sea level fall and during the earliest rise. If sea level drops below the shelf edge, rivers will cut into the exposed surface and allow sediments to by-pass the shelf. Thus, submarine fans form at the base of the continental slope (i.e. on the continental rise). Eventually, relative sea level begins to slowly rise due to either a slowing rate of sea level fall, which allows subsidence to outpace the falling sea level, or the beginning of a sea level rise. In this case, the incised river valley may fill with deltaic sediments, which prograde out to the shelf edge to feed the still growing submarine fans with gravity deposits (turbidites!).

c. Transgressive Systems Tracts: Formed during a rapid sea level rise that floods the shelf. Beach and inner shelf sediments move landward in parallel with the advancing shoreline. The outer shelf tends to be relatively sediment starved during this phase of rapid rise and thus accumulates "condensed section deposits" (Boggs Fig. 15.2). These include such things as limestone or phosphatic hardgrounds and/or thin shales hosting phosphatic nodule horizons and/or high concentrations of horizons of bored fossil materials.Following this period of rapid sea level rise comes a slower period of sea level rise, then stillstand, then the start of the next fall. During this period, beach, shelf, and fluvial/deltaic sediments can prograde seaward to produce the next Highstand Systems Tract (see above). If the next sea level fall is not as fast or far, instead of a lowstand systems tract, we get the following

d. Shelf-Margin Systems Tracts: With a slow rate of fall, fluvial, coastal plain, and delta plain sediments prograde seawards across the highstand systems tract. At the shoreline, sediments rapidly fill the accomodation space such that aggradation is as important as progradation. G. Sequence Stratigraphy and Eustatic Sea Level. When Peter Vail and his associates at Exxon developed sequence stratigraphy in the 70's, they assumed that eustatic (global) sea level changes were the main factor influencing the geometry and timing of systems tracts and sequence boundaries. Local tectonic (uplift (+erosion) and subsidence) and climate (erosion rates) variables were assumed to be far less significant than global sea level. As a result, they figured that they could generate a high-resolution global sea level curve from their data. Handout: Boggs Figs. 15.4, 15.5, 15.6 Without going into much detail, they looked at (Unfortunately, most of these data were locked up in the proprietary archives of Exxon and thus could not be independently evaluated by other geologists. This many found extremely irritating.) People seem to be generally happy with the first order sea level curve (several major fluctuations over the last 500 million years) and the general second order sea level changes (changes over 10s to 100 Myr), many people do not think that the third order sea level variations (10 to a few Myr) are global. Although this interpretation of sequence stratigraphy in terms of global sea level variations is hotly disputed, it is true that many many people, especially those exploring for oil, find that sequence stratigraphy is an extremely useful way for understanding the local and regional history of a basin. It is only the global extrapolations that seem a bit off.

Stratigraphy ( Biostratigraphy )

II. Biostratigraphy:

 Two most important things.

A. Biostratigraphy is the single most potent way we have of telling geologic time. It allows us to figure out which rocks are the same age as which other rocks, and thus allows us to piece together the rocks that assemble a given depositional environment. Without fossils, we could only rely on the laws of superpositioning and cross-cutting relations to work out the regional geologic history of a given area. These generally do not offer enough constraining observations to go beyond is generally a very broad impressions of the geologic past of a given area.

B. The main problems in biostratigraphy are as follows:

1.       Not all fossil groups are equally useful. Correlations based on groups that slowly evolve or whose presence depends on the nature of the substrate (for example, clams, snails, brachiopods) may not be nearly as precise and accurate as freely swimming groups such as coccoliths, foraminifera, and the ammonites.

2.       Good fossils can be hard to find. Some sequences yield only a few fossils, and these few relatively poor time constraints may allow several equally valid correlations that dramatically affect how you interpret the paleoenvironment. It is important to understand the limitations of the data so you recognize those cases in which more data are critical.

Stratigraphy ( Lithostratigraphy )

I. Lithostratigraphy

     Actually determining that a sandstone seen at two outcrops a mile apart is part of the same formation. In the desert, where you have great exposure, you can often be pretty sure of a good correlation. Around Pittsburgh, one sandstone looks much like the next, and the exposures are too limited to see much of the rocks that occur above or below (stratigraphic context can be extremely helpful). Also, in a humid climate, a fresh exposure is often a very different color from an old weathered one. So, don't underestimate the difficulties of correlating rocks based on their physical appearances. This is one reason that biostratigraphy becomes so important

B.   Rocks do not equal time. On the scale that we see outcrops, either in road cuts or across whole mountain sides, many sedimentary formations show up as nice layers with more or less parallel tops and bottoms. Almost every outcrop we see, even if it crosses a whole mountain side in the desert, looks like a beautiful layer cake. The layers were clearly originally horizontal, with the oldest on the bottom and the youngest on the top, and they appear to be quite laterally continuous. If you accept this impression as fact, it perverts everything you have learned in class thus far. Handout: Trangression/Regression general shelf box drawings. There are 3 points: First, at the scale of the drawing, it is clear that the sandstone beds in fact dip toward the sea. HOWEVER, this dip is obvious ONLY because of the extreme vertical exaggeration of the drawings. Recall that the shelf dips on average only 0.1° seaward. As far as your eyes are concerned, this is in fact horizontal. But, when gazing at a given outcrop, remember that the layers are really generally dipping toward the center of the basin. Second, a given formation can span a considerable time range. While this is obvious when it comes to considering a single vertical section (old on bottom, younger on top), it is harder to keep this in mind when you consider the age of the base and top of a formation as you walk over a wide area. On these figures, the passage of time is marked by time lines, which define ancient active depositional surfaces. Lines or surfaces like these are termed "isochronous. Looking at the regression/transgression figures, you can see that the bases and tops of formations start and end at different times. If you were measuring sections around the basin, you would find sequences of layered beach sands deposited immediately above an underlying formation. They might all look very similar, because they each represent the same depositional environment,  if you look at the figures, you can see that they will be of different ages that depend on the nature of the transgression or regression. The only way that you can be sure that a set of sand layers was deposited at the time (i.e., along strike parallel to the shoreline) is by using biostratigraphy to constrain the relative ages of the strata. This biostratigraphic time frame will allow you to put the basal sand units, or whatever rock bodies you want to consider, into their proper depositional frame work. Notice how the overall thickness of the formations varies depending on whether they are parts of transgressive and regressive sequences. Third, once time lines are established, you know that if you follow them into deeper water, you will go from sand (beach) to silt to clay (shelf). If you stay at a given water depth and move parallel to the shore line, you can stay within a given sandstone formation at the same time horizon. Handout: Devonian Catskill sequences of New York. Both figures are based on a compilation of vertical sections measured at a number of localities across southern New York. If you are thinking 'layer cake', your interpretive cross section will look like the upper drawing. Note how the Catskill Group appears to have just somehow rained down from Heaven to lay nicely on the underlying formations. It is tough to reconcile such a cross-section with an understanding of how sedimentary environments work. The lower drawing illustrates how, with a better prior understanding of depositional systems and how sedimentary facies change through space and time, the formations are now seen as a part of a prograding sequence. Biostratigraphy is necessary to draw the time lines that help estimate the ancient depositional surfaces. In this example, the base of the sequence has several limestone beds that indicate outer shelf sediments. Shelf shales come next, apparently pushed in from the east. As the available accommodation space fills, the sediments coarsen up to beach facies. Finally a sequence of redbeds indicating continental deposition sweep west across the area.

C.    When faced will real outcrops in the wild, it is a challenge to force yourself to not see the world as a layer cake. Outcrops just look like layer cakes because their original depositional dips are so shallow!

D.    Sedimentary rocks are not a continuous record of time! They are full of time gaps when no sediment was deposited and preserved or when preserved sediment was later eroded.

1.    Ancient record: measure thickness of 5, 10, or 100 Myr sequence of sedimentary rocks, divide by the total age range they represent, and sedimentation rates are meters/ka. Modern environments: sediments accumulate 100 to 1000 times faster (100 to several 1000 meters/ka). Clearly, a great deal of sediment accumulating today will not make it into the sedimentary rock record. There is a lot of erosion! From calculations like those above, geologists conclude that only <0.1% to 1% of geologic time is typically represented by sedimentary rock. This is true even in the deep sea. One wonders if this 0.1 to 1% of sediment is a representative sample of sediments originally deposited in the ancient sedimentary environments, or if it all represents extraordinary events like huge rare storms.

2.   Example: Beach sediments. Over the course of 6 months, sediments are during fair weather, then small storms strip some off, more fair weather deposition follows, big storm strips off a lot of beach, then a long period of fair weather deposition follows, etc., etc. Maybe only tiny snippets of longest periods of fair weather deposition will survive to next season. How to get beach into ancient record? An exceptionally big storm (once every 1000 or 10,000 years?) sweeps beach sediments into deeper water, where they can get buried? Perhaps a big (HUGE) earthquake drops the beach surface down enough meters to preserve much of it below wave base?

3.    Unconformity: a much longer break in the sedimentary record (generally one to multi-Myr time scale). Often marked by erosion (as opposed to nondeposition), and generally caused by significant changes in the sedimentary regime (as opposed to normal processes occurring at at given environment). These gaps in time can be estimated using stratigraphic techniques. The shifts in sedimentary environments may be caused by a major sea-level changes or by regional uplift and erosion.

Petroleum System

A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porousrock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability. Reservoirs are found using hydrocarbon exploration or fractured methods.

Formation

Crude oil found in oil reservoirs forms in the Earth's crust from the remains of living things. Crude oil is properly known as petroleum, and is used as fossil fuel. Evidence indicates that millions of years of heat and pressure changed the remains of microscopic plant and animal remains into oil and natural gas.
Roy Nurmi, an interpretation adviser for Schlumberger, described the process as follows: "Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70 °C they start to cook. This transformation, this change, changes them into the liquid hydrocarbons that move and migrate, will become our oil and gas reservoir.
In addition to the aquatic environment, which is usually a sea, but might also be a river, lake, coral reef or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps: deep burial under sand and mud, pressure cooking, hydrocarbon migration from the source to the reservoir rock, and trapping by impermeable rock. Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps. The North Sea, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oilfields.
Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land surface to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and ages.

Traps

The traps required in the last step of the reservoir formation process have been classified by petroleum geologists into two types: structural and stratigraphic. A reservoir can be formed by one kind of trap or a combination of both.

Structural traps

Structural traps are formed by a deformation in the rock layer that contains the hydrocarbons. Domes, anticlines, and folds are common structures. Fault-related features also may be classified as structural traps if closure is ‎present. Structural traps are the easiest to locate by surface and subsurface geological and geophysical studies. They are the most numerous among traps and have received a greater amount of attention in the search for oil than all other types of traps.
An example of this kind of trap starts when salt is deposited by shallow seas. Later, a sinking seafloor deposits organic-rich shale over the salt, which is in turn covered with layers of sandstone and shale. Deeply buried salt tends to rise unevenly in swells or salt domes, and any oil generated within the sediments is trapped where the sandstones are pushed up over or adjacent to the salt dome.

Stratigraphic traps

Stratigraphic traps are formed when other beds seal a reservoir bed or when the permeability changes (facies change) within the reservoir bed itself. Stratigraphic traps can form against either younger or older time surfaces.

Estimating reserves

After the discovery of a reservoir, a petroleum engineer will seek to build a better picture of the accumulation. In a simple text book example of a uniform reservoir, the first stage is to conduct a seismic survey to determine the possible size of the trap. Appraisal wells can be used to determine the location of oil-water contact and with it, the height of the oil bearing sands. Often coupled with seismic data, it is possible to estimate the volume of oil bearing reservoir.
The next step is to use information from appraisal wells to estimate the porosity of the rock. The porosity, or the percentage of the total volume that contains fluids rather than solid rock, is 20-35% or less. It can give information on the actual capacity. Laboratory testing can determine the characteristics of the reservoir fluids, particularly the expansion factor of the oil, or how much the oil expands when brought from high pressure, high temperature of the reservoir to "stock tank" at the surface.
With such information, it is possible to estimate how many "stock tank" barrels of oil are located in the reservoir. Such oil is called the stock tank oil initially in place (STOIIP). As a result of studying things such as the permeability of the rock (how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to estimate the recovery factor, or what proportion of oil in place can be reasonably expected to be produced. The recovery factor is commonly 30-35%, giving a value for the recoverable reserves.
The difficulty is that reservoirs are not uniform. They have variable porosities and permeabilities and may be compartmentalised, with fractures and faults breaking them up and complicating fluid flow. For this reason, computer modeling of economically viable reservoirs is often carried out. Geologists, geophysicists and reservoir engineers work together to build a model which allows simulation of the flow of fluids in the reservoir, leading to an improved estimate of reserves.