Chatellier, J-Y.,1 Campos, O., and 2 Porras J. C.2
Search and Discovery Article #40062 (2002)
*Adapted for online presentation from poster session presented at the AAPG Convention, Houston, Texas, March, 2002.
1Consultant to PDVSA Intevep ([email protected])
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New techniques making use of petrophysical data were introduced in a full blown integrated study in order to decipher the sedimentological and structural complexity of the Santa Barbara Field. Understanding and validating the sedimentology proved difficult because of local folding invoking high angle dips and because of the existence of numerous thrusts and detachment faults. Isochore maps are thus highly disturbed by the structural complexity of the area. Three newly defined methodologies, based on a statistical analysis of petrophysical averages have shed a new light on the Santa Barbara Field. These are based on a semiquantitative quick-look dip evaluation using net-to-gross derived decompacted isochores and on 3-D visualization of porosity and water saturation depth trends.
Net-to-Gross maps are very useful to understand and review sedimentary environments, morever, N/G values can also be used as a decompaction factor in order to control the quality of a stratigraphic correlation. A Quick-Look dip evaluation method has been devised using the ratio between the decompacted thickness of a unit and the equivalent decompacted thickness in a reference well. In the Santa Barbara Field, the Quick-Look dip evaluation has corroborated the existence of large folds and of local detachment planes that have altered the apparent thickness of the unit and dramatically reduced the reservoir quality. Cores have confirmed the proposed hypotheses.
The traditional but very powerful Porosity Depth Trend Analysis gave new insight into the structural complexity of the area and confirmed that local reservoir quality deterioration is not linked to lateral facies changes but is due to tectonically derived processes.
In the complex Santa Barbara Field, one of the petrophysical problems encountered is the evaluation of water saturation, due to uncertainties in the estimation of true formation resistivity. Exploratory statistics (3D) complemented by a depth trend analysis has clearly shown that the exaggerated saturation values are due to higher dip in thinner bed intervals. The maps of water saturation anomalies are in total agreement with the structure of the field and can be used to predict zone with highly dipping beds and to correct saturation maps.
Figure 3. Porosity-depth trends superposed on structure map, Santa Barbara Field. See Figure 17 for detailed plots.
In order to understand the sedimentological and structural complexity of the Santa Barbara Field (Norte Monagas, Venezuela - Figure 1) a few new techniques were introduced in a full blown integrated study. This field is a large compressional structure producing from Oligocene and Cretaceous sands, with more than 150 wells to date. The hydrocarbon column is complex and of the order of 2500 to 3000 feet and produces about 240,000 BOPD (Embid et al., 2001). The following paragraphs summarize the use of petrophysical trend analysis as a tool to better define the stratigraphy, the sedimentology and the structure. Understanding and validating the sedimentology has proved difficult because of local folding invoking high angle dips. Isochore maps are thus highly disturbed by the structural complexity of the area. Three newly defined methodologies based on a geostatistical analysis of petrophysical averages have shed a new light on the Santa Barbara Field. These are based on a visualization of porosity and of a qualitative Quick Look. Dip Evaluation using net-to-gross derived decompacted isochores.
An example for each of the three methods (Quick-look petrophysical dip evaluation, porosity depth trend analysis, and water saturation depth trend analysis:is shown in Figures 2, 3, and 4; the description and results are presented in the following parts of this article.
In tectonically complex areas, sedimentary units may have moved a long distance over or along other strata; this is especially true in compressive and transcurrent tectonic regimes. As a result the present day lateral juxtaposition of facies may be very different from the original one, bringing in some cases shale intervals next to coeval sandstone intervals. Moreover, a change from sand to shale can also occur as a simple lateral facies change; the large distance between wells in sparsely drilled areas may thus show dramatic facies changes between wells.
The idea of making a simple decompaction correction is that soon after deposition muds are compacting very rapidly because of the gradual expulsion of the trapped water. On the other hand, any neighboring sand will barely compact because of their grain supported nature. The simple formula used here can be modified at will (more details on compaction can be found in De Waals (1986), Giles (1997) and Fisher et al. (1999). The aim of decompacting is to make sure that we do not to falsely interpret high dips when the comparison is made between sands that will have undergone only some decompaction and shales that will have decompacted a lot (Figure 5).
There is no essential need to correct for the compaction in cases where lateral facies changes are not dramatic; however, a correction is needed if we expect lateral changes from sandstones to shales (e.g., in turbidites, anastomosed river setting or in some tectonically complex areas).
Decompacted thicknesses have been estimated using the net-to-gross value as a correction factor. It is based on a simple formula that takes into account a decompaction factor for a pure sand, a decompaction factor for a pure shale, and the net-to-gross value of the interval considered (Figure 6).
The formula used has been devised such that the correction factor will fluctuate with the value of Net-to-Gross. Different sets of extreme values for the correction might be chosen to reflect variations from one basin to another or from one formation to another.
The use of a gradual fluctuating factor based on the net-to-gross allows a comparison between sands, shales, and interbedded sand-shale sequences in one single exercise.
Comparing decompacted thicknesses is vital to validate a stratigraphic correlation; thus after decompaction a shaly unit should not be thicker (or very little thicker) than the neighboring coeval sandy unit if no fault is present between these wells. Anomalies of decompacted thicknesses should be systematically reviewed before making isochore or isopach maps.
The association of a crossplot of thickness ratios versus geographic position (Figure 7) and of a map (Figure 8) allows one to understand some of the observed thickness anomalies. The abnormally low values (colored circles in a green/yellow trend) correspond to missing sections due to faulting.
General structural geometry can be easily defined by the “Quick-Look petrophysical dipmeter”:
The chosen example for the Quick-Look dip Evaluation is the southwestern part of the Santa Barbara Field (Figure 9). The reference for decompacted thickness has been derived from statistics and has been chosen as the 25 percentile value of all of the wells belonging to Santa Barbara. Note that, in the present case, one single reference well was used for the whole field as a first approach; that has implied the use of statistics. A reference well can be chosen deterministically when working exclusively on a small,well defined area. Despite the fact that the reference value is not the most suitable for the present example due to local variations, there is a perfect trend of thickness increase towards the West (Figure 10). Thus, for the selected stratigraphic unit, all but four of the wells in the area of interest (Figure 11) fall on a linear trend. The trend is too good not to be meaningful.
The core observations and the Quick-Look dip evaluation can be simply understood within a simple fold associated with a detachment plane, the latter increasing the measured thickness because of repetition (Figure 12). In one of the anomalous wells (well D), the interval under study, which was cored, exhibits horizontal faulting and fracturing over some 40 meters (Figures 13 and 14).
Note that without understanding the folding and faulting described here and derived from the Quick-Look dip evaluation method, an isochore map would have indicated a north-south trending channel direction, whereas the thickening is due to folding and repetition by a low-angle fault.
There is a thickness increase due to folding, but also, as illustrated in Figure 12, there is a thickness increase due to repetition linked to a detachment at 15,900 ft. The cores show some 40 meters of intense fracturing around the detachment plane at 15,900 ft TVDss (Figure 13). Note the high density of low-angle and horizontal open fractures (Figure 14).
Other types of observations corroborate the validity of the proposed model based on the quick-look dip evaluation and of the method used (Figures 15 and 16). Figure 15 shows changes in thickness due to folding and faulting.
Variations of porosity with depth are observed in the Santa Barbara Field because a large part of the tectonic activity is posterior to the hydrocarbon emplacement.
The rock quality is linked to the burial and diagenetic history prior to the emplacement. Areas of the field are characterized by well defined trends that are linked to the tectonic overprint (Figures 17, 18, 19, and 20).
The three shallowest wells in Figures 18 and 19 exhibit linear porosity-depth patterns. Well X, all of well Y except the lowermost unit, and the lowest two units of well C are perfectly aligned. This group of data points is parallel to another trend (Trend 2) formed by 4 points from well C (Figures 18 and 19). Two anomalous points correspond to a zone affected by the detachment seen with the Quick-Look dip evaluation and confirmed by core observations.
In the complex Santa Barbara Field, one of the petrophysical problems encountered is the evaluation of water saturation, due to uncertainties in the estimation of true formation resistivity. Conventional petrophysical evaluation of the induction logs may result in abnormally high values of water saturation.
The problem has been found in shaly sands as well as in clean sands, whereas nearly no water has ever been produced from the field. Exploratory statistics (3D), complemented by a depth trend analysis, has clearly shown that the high saturation values are in part due to a combination of existence of higher dips and of more shale interbeds.
A depth trend analysis of the saturation was performed on the whole data set and for each stratigraphic unit; the saturation values used correspond to the arithmetic averages of the water saturation. The data distribution and our study seem to indicate that, if a cut-off were to be applied, it would have to be a function of depth and it should not be a single value.
A cut-off was chosen graphically on the whole data set and verified by each of the individual stratigraphic units. By attributing two colours to the two populations of data located on each side of the cut-off line (Figure 21), we were able to evaluate the validity of the hypothesis by studying the map distribution of these two populations for every single stratigraphic unit. Note that the choice of cut-off is essentially directed towards visualizing the problem at hand.
All of the observations can be summarized in a few diagrams comparing Unit 6 and Unit 7 from Santa Barbara Field (Figure 21). The box plot diagram of Net-to-Gross per unit clearly shows that Unit 7 corresponds to the top of a fining-upward sequence with abundance of shale intervals and that unit 6 is the base of a new fining-upward sedimentary cycle and is much more sandy in nature. In Unit 6 only three measurements have abnormally high water saturation, whereas many points are too high in Unit 7. The geographic distribution of the abnormally high values in Unit 7 is in complete agreement with the structure of the field with the abnormally high saturation values located in the zones of high dips in the recognized structure (Figures 22, 23, and 24).
The maps of water saturation anomalies are in total agreement with the structure of the field and can be used to predict zones with highly dipping beds. In zones of low dips, no anomalies are found.
Figure 25 shows an outcrop analog to illustrate the problems related to the existence of anomalously high water saturation measurements in alternating sandstone-shale sequences and, more particularly, when the sandstones beds are thinner.
The error related to the saturation is increased with an increase in dip as well as with a decrease in thickness of the sandstone beds. The problem is essentially due to the way an induction log operates (Anderson et al., 1995; Allen et al., 1997).
The statistical identification of the abnormally high water saturations in the low Net-to-Gross intervals and the understanding of their geographic distribution will help pinpoint where beds may be highly dipping.
These findings can be extrapolated for the sandier intervals where saturation may be correctly measured but where the thickness is abnormally high because of the dip; in such cases this information can be used to make isopach-corrected thickness maps, even if the correction is only semi-quantitative.
Finally, the values of dips so obtained may be input into more advanced petrophysical modeling applications.
Net-to-Gross maps are very useful to understand and review sedimentary environments, additionally N/G values can be used as a decompaction factor in order to quality control a stratigraphic correlation.
A Quick-Look dip evaluation has been devised using the ratio between the decompacted thickness of a unit and the equivalent thickness in a reference well. In one part of the Santa Barbara Field, the Quick-Look dip evaluation has corroborated the existence of a large fold and of some local detachment planes that have altered the apparent thickness of the unit and dramatically reduced the reservoir quality.
The traditional but very powerful Porosity Depth Trend Analysis gave new insight into the structural complexity of the area and has confirmed that the reservoir quality deterioration is not linked to lateral facies changes but are due to tectonically derived processes. The scale of the structural complexity identified includes features not resolved by 3D seismic mapping.
Visualization of petrophysical averages in 3-D has enabled a much clearer picture of the thickness, porosity, and water saturation variations and has given a set of new tools that will help understand better the structure of the field. These new tools identify the zones where petrophysical parameters need to be corrected for the influence of folding and faulting.
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We would like to thank P.D.V.S.A. for permission to publish this work and to Omar Colmenares, James Helwig, Luis Brito, and Elizabeth Sampson for their critical review of the poster.