This reference is for an abstract only. A full paper was not submitted for this conference.
Discovery of oil in a recent well has added a Channel complex of the mid-Miocene, Lower Fars Formation to hydrocarbon bearing formations in northern Kuwait. In northern Kuwait the Lower Fars Formation is a long established, prolific reservoir. Initiation of this study was prompted by observation of oil on the shaker combined with a ‘gas kick’ on the mud-log whilst drilling a recent well on the Sabriyah field in northern Kuwait, Figure 1. SA-ChC is the only well in the area to have recovered mud-log chromatography records from this shallow level. Older wells have penetrated the Lower Fars Formation without the benefit of an operating chromatograph. Nevertheless, they provide information on observations of oil shows and on the formation lithology at corresponding stratigraphic levels.
The Channel complex is referred to the basal parts of the Lower Fars Formation; possibly as a lateral equivalent to the F2 sands at Ratqa Oil Field. Stratigraphic relationship is based on its position relative to the Top Ghar Formation as interpreted from Gamma Ray logs.
Examination of seismic data at the location of the well indicated good correlation between the level of the observed oil at 670 ft. KB (~145 msec. TWT) and a lenticular, high-amplitude anomaly in the deeper parts of the Lower Fars Formation. Detailed study suggested that the anomaly represents the channelised fill of an incised valley cut up to 200 ft into the host formation. The valley extends some 35 km south from the Iraqi border in a broad easterly meander. Width varies between 3 and 6 km, Figure 2.
Mapping of the channel system has been by means of seismic horizon-consistent slices and volume attributes extracted over a variety of window apertures. Best results were obtained with amplitude attributes, supported to some extent by frequency and phase. Study of seismic attributes indicates both vertical and horizontal segmentation into individual channel lobes. The youngest lobe identified is near well ChC in the south, thus indicating back-stepped Channel deposition in a north to south direction. The seismic horizon, ‘Top Channel Envelope’ was picked in a conventional manner on vertical amplitude sections. It is the nearest continuous seismic event identified above the channel development, Figure 2. Primary objective for its mapping was to provide a reference horizon for subsequent attribute extraction work, rather than to construct a detailed structure map. The interpreted horizon also provided an approximation to post-depositional structural deformation, Figure 1. Lower Fars Formation hydrocarbon potential has been confirmed in several shallow wells drilled outside the Channel complex. Deep, wide-bore wells only have penetrated the complex to date, and useful data at this stratigraphic level is very limited. Offset horizon slice attributes are limited to Amplitude as the only extractable seismic trace attribute. Horizon consistent Amplitude slices were produced at 8 msec. intervals below the ‘Top Channel Envelope’ horizon. Parts of the Channel complex outline were clearly defined, but general consistency was thought to be insufficient for detailed mapping. Offset volumes, on the other hand, offer a wider choice of Amplitude attributes in addition to Frequency and Phase. Response patterns were noted to be distinct and consequently adequate for the confident mapping of discrete anomalies. Best results were attained with the RMS and Maximum Trough Amplitude attributes. Other Amplitude attributes corroborated or provided information complementary to the two primary attributes selected. Average Instantaneous Frequency and Phase were less distinct, but appeared to include indications of lateral compartmentalization of the Channel complex additional to what is visible on the Amplitude attributes panels studied, Figure 3. The complementary nature of RMS and Maximum Trough Amplitude is illustrated in Figure 3. Within the Channel complex outline, the two attribute responses are very similar in the 64 msec. window shown, thus confirming the predominance and distribution of a high-amplitude, negative seismic response. By comparison Average Trough Amplitude is less distinct than Maximum Trough amplitude, and Peak Amplitude displays are inconclusive. Frequency and Phase attributes are sensitive to the number of samples included compared to the vertical extent of the investigated anomaly. If the ratio is large, then the response becomes contaminated by surrounding events with consequent poor imaging of the target zone. The Instantaneous Frequency panel in Figure 3 shows low correlation with the Amplitude panels, Figure 3, Attribute volume window 0–64 msec. below ‘Top Channel Envelope’ horizon. Seismic attributes and Channel complex outline. Location of well SA-ChC. whereas the Phase display shows good correlation, albeit with partially greater resolution of detail along central and southern parts of the Channel complex. In the north, however, there is no Phase anomaly associated with the Channel complex outline as indicated on the other three attribute panels in Figure 3
Horizon consistent attribute analyses - Window apertures: Judging from vertical sections, maximum thickness of the Channel complex is approximately 64 msec. Attribute panels reproduced in Figure 3 illustrate the response for the full 64 msec. window aperture. RMS and Maximum Trough Amplitude response show distinct anomalies, whereas Frequency and to some extent Phase are inconclusive. 16 msec. attribute volume windows are reproduced in Figure 4. Differences in attribute response are considerable among the panels and indicate both lateral as well as vertical compartmentalization of the Channel complex. RMS and Maximum Trough Amplitude response in the shallower 32 msec. section is completely different and reflect the composite response polarity which includes very low trough amplitudes. These values are not recorded as anomalous by the RMS processed attribute, whereas they are clearly visible on the Maximum Trough display. Frequency and particularly Phase response support the interpretation of the Channel complex's outline. It is augmented by the Frequency and Phase responses, which indicate a possible southerly extension beyond the outline of the amplitude attributes. A gradual upwards decrease in response frequency accompanied by phase rotation relative to surrounding, host beds is observed within the Channel fill in the south, Figure 4. In the deeper section in the north, the channel outline is defined by an abrupt change in frequency, which, however, remains high compared to the shallow section. Phase response becomes increasingly distinct in smaller aperture windows. Systematic vertical and lateral changes in attribute response are interpreted as manifestations of different Channel lobes of potentially different lithological compositions. Figure 5. Spectral Decomposition of the full, 64 msec. Channel complex section reveals strong variation in frequency distribution. Low frequencies trace the length of the Channel complex, whereas higher frequencies are limited to the northern parts only. Numerous wells targeted on the Lower Fars Formation have been drilled in the area, but only a few have penetrated the Channel complex proper. Marginally representative penetrations have been along the edge of the complex at its southernmost extremity where the complex is thin. Presence of hydrocarbons, heavy oil 12–18 API, has been confirmed in area, and within the Channel complex itself as indicated in the recent SA-ChC well. Subject to favourable reservoir parameters, the Channel complex could add a substantial, clearly defined reservoir volume to the previously discovered, major oil reserves identified elsewhere in the Lower Fars Formation in Kuwait.