Thermal Maturity Evaluation PDF

THERMAL MATURITY EVALUATION Thermal alteration index (TAI) ‫(تغییر رنگ پوسته‬ )‫میکروفسیلها‬ Vitrinite Reflectance Measurement (%Ro) Tmax derived from Rock-Eval 34  Thermal Alteration Index (TAI) ▪ TAI is a maturity indicator based on obs...

THERMAL MATURITY EVALUATION Thermal alteration index (TAI) ‫(تغییر رنگ پوسته‬ )‫میکروفسیلها‬ Vitrinite Reflectance Measurement (%Ro) Tmax derived from Rock-Eval 34  Thermal Alteration Index (TAI) ▪ TAI is a maturity indicator based on observations of the progressive change in the color of spore and pollen particles in kerogen with increasing maturity. ▪ The first formal scale was developed by Staplin (1969), and it used a 1–5 scale employing + and − notations to signal intermediate steps, as shown below. 35 Examples of colors under microscope 36  Maceral ▪ A maceral is a component, organic in origin, of coal or oil shale. ▪ Examples of macerals are inertinite, vitrinite, and liptinite. 37  Maceral equivalent to mineral in rock SEM-BSE image of a sandstone mapped with Automated Mineralogy Microscopic image of vitrinite maceral (Ro=0.61%) in reflected white light (left) and fluorescence mode (right); 50X. 38  THREE MAJOR MACERALS: Vitrinite, Inertinite and Liptinite ▪ LIPTINITE is derived from hydrogen-rich plant material and decomposition products (e.g. decayed spores, pollen and algal matter). ▪ VITRINITE is derived from land plants ▪ There has been considerable controversy over the origin of INERTINITE, which shows high reflectance, little or no fluorescence, high carbon and low hydrogen contents, and strong aromatization. 39 40  VITRINITE REFLECTANCE ❑A measurement of the maturity of organic matter with respect to whether it has generated hydrocarbons or could be an effective source rock. ❑In fact, vitrinite reflectance is a measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock. ❑It is referred to as %Ro 41  VR methodology ❑This analytical method was developed to rank the maturity of coals and is now used in other rocks to determine whether they have generated hydrocarbons or could be effective source rocks. ❑The reflectivity of at least 30 individual grains of vitrinite from a rock sample is measured under a microscope. ❑The measurement is given in units of reflectance, % Ro, with typical values ranging from 0% Ro to 3% Ro, with values for gas-generating source rocks typically exceeding 1.5%. 42 43 Ideally, the vitrinite reflectance trend should show the data increases with increasing depth as a straight line with a surface intercept for the trend between 0.20% and 0.23% Ro 44  There are many possible interferences with the vitrinite trend due to several reasons such as: ❑ Lack of vitrinite ❑ Caved sediments ❑ Reworked organic material ❑ Faults ❑ Unconformities ❑ Thermal events ❑ Suppression/misidentification Vitrinite reflectance trends with depth do not always exhibit the characteristics of the “ideal” trend. 45  Lack of Vitrinite ❑Not all sediments contain vitrinite. Some depositional settings may simply not receive significant contributions from higher plants, resulting in little or no indigenous vitrinite. ❑Because of its higher plant origins, vitrinite should not exist in pre- Devonian sediments in the lower Palaeozoic and Precambrian. ❑If there is no vitrinite present, its reflectance cannot be measured, but this sometimes leads to measurements being made on kerogen particles with “vitrinite-like” appearances 46  Caved Vitrinite ❑During the drilling of a well, up-hole sections may become progressively unstable due to physical and mineralogical characteristics of the rock and the rock’s interaction with the drilling fluid. This can result in up-hole intervals caving, or sloughing off, into the borehole, contributing lower maturity vitrinite particles to a cuttings sample. ❑If core or sidewall core material are not available, cuttings samples collected below just casing points are often used instead. Casing cuts off any potential caving from above making samples just below the casing point desirable for sorting out caving problems. 47  Reworked Vitrinite In the case of reworked vitrinite, pre-existing source rocks can be eroded and redeposited, thereby contributing potentially higher maturity vitrinite particles to a sediment. 48 49 Faults 50  Unconformities/Normal Faults ▪ The loss of section brings higher maturity sediments to be juxtaposed to lower maturity sediments at the unconformity surface ▪ Unconformities that occur at the surface will also impact the vitrinite reflectance trend. With loss of sediment at the surface due to erosion, the surface intercept of the vitrinite reflectance trend will not be in the vicinity of 0.20–0.23% Ro, but rather at a higher value. ▪ Similar patterns are observed with offsets in vitrinite reflectance trends due to normal faulting. 51 unconformity 52 Reverse Faults ▪ Displacement of sediments by a reverse, or thrust, fault will move higher maturity sediments up and over lower maturity sediments. This will result in an offset in the vitrinite reflectance trend. ▪ It has been suggested that offset, or vertical displacement, of the fault can be estimated from the vitrinite reflectance trend. However, the estimated offset should only be considered a minimum due to the process of annealing. 53 Reverse Faults 54  Igneous Intrusion ▪ Localized high-temperature heating by an igneous intrusive can cause a high-maturity excursion in the vitrinite trend. ▪ Thermal effects of an igneous intrusive typically extend out to a distance equal to about twice the thickness of the intrusive on both sides of the igneous body 55 56  Suppression and Misidentification ▪ Occasionally, there is a localized decrease in reflectance values in a vitrinite trend with depth that is usually confined to an interval corresponding to a rich oil-prone source rock. ▪ One explanation for this phenomenon is called suppression. This occurs when oil generated in rich oil-prone source rocks supposedly invades vitrinite particles under subsurface pressures and lower their reflectivity. ▪ While suppression may affect sedimentary organic matter, misidentification of vitrinite like particles should not be overlooked as a possible cause for localized lower than expected reflectance values. ▪ Rich oil-prone source rocks often contain little or no indigenous vitrinite, and this lack of vitrinite may lead to misidentification of other kerogen particles with vitrinite-like appearances, such as solid bitumen. 57 Very rich oil-prone source 58 Positions of the oil and gas windows will vary depending on kerogen type, but typically are in the following ranges: 59 60 61 Interpretation ❑Richness ❑Maturity ❑Rock-Eval 62 Geochemical logs 63  Source Richness Interpretations based on the Rock-Eval S1 and S2 parameters ❑S1 representing the hydrocarbons that have already been generated ❑S2 represents the hydrocarbons generating potential remaining in the sediment’s kerogen. ❑Current drilling practices rely heavily on oil-based drilling mud and a large variety of organic drilling mud additives. These materials can contribute significant amounts of material to the S1 peak and obscure its true value. As a result, the S1 peak has fallen out of favor as a source richness indicator. 64  S2 is often combined with TOC data in a cross-plot Assuming they have a similar thermal maturity, this likely indicates the two source rocks have two difference kerogen type mixtures. 65  Total organic carbon (TOC)-S2 cross-plot showing isohydrogen index lines The two parallel data trends have different average hydrogen indices which suggests different mixes of kerogens: the source rock represented by the diamond symbols being more oil prone, and the source rock plotted with the triangles being gas prone-to-inert. 66  S2 - Pit falls! ❑While the S2 is supposed to represent the hydrocarbons generating potential remaining in the sediment’s kerogen, there are instances when some of the material that should be confined to the S1 are carried over and included in the S2. This can occur when a high proportion of the generated material consists mainly of resins and asphaltenes that may not be volatile at 300°C. These materials would be volatilized at higher temperatures and thereby be included in the S2 peaks. ❑A similar situation can occur with contamination of the sediments by organic drilling mud additives. Some of these additives may contain or consist of processed asphalt or Gilsonite that will contribute to the S2 peak ❑To recognize the presence of resins and asphaltenes in the S2 peak either from the bitumen or drilling mud additives, it is necessary to examine the pyrograms from the Rock-Eval analysis. These contributions to the S2 peak can be recognized as an asymmetry in the peak on the low-temperature side. 67 Another caution in using Rock-Eval S2 for a source richness indicator concerns what happens to S2 as the source rock matures 68 Thermal maturity VS TOC ▪ Thermal maturity will reduce TOC values depending on kerogen type. ▪ As a rule of thumb TOC values will be reduced approximately: ✓ 70% Type I ✓ 40% Type II ✓ 20% Type III 69 TOC For a present-day high thermal maturity, Type II shale showing Ro=1.6% would have its TOC reduced by 40% so its original TOC would be: TOCpd/0.60 e.g. a 5% TOC present-day at 1.6%Ro on a Type II kerogen would have about 8.3% original TOC or about 3.3% of carbon converted to hydrocarbon. 70  Thermal Maturity Interpretations While Tmax values for Type II kerogen will be approximately 435°C at the top of the oil window, Type I kerogen will have Tmax values of about 440°C. Significant hydrocarbon generation in Type III will be indicated by Tmax values of about 445°C. As maturity progresses, the kerogen type influence diminishes, and all three kerogen types have Tmax values of approximately 470°C at the bottom of the oil window. 71 ❑Further complicating the problem, the S2 peak can occasionally have multiple maxima making it difficult to pick an appropriate Tmax for the analysis ❑In contrast, when the S2 peak is small,