Avustralya Fıtzroy Havzasında Çok Kanallı Sismik Yansıma Verilerinin İşlenmesi Ve Yapısal, Stratigrafik Değerlendirme Ve Gaz Varlığının Araştırılması

Abstract

Bu çalışmada Batı Avustralya eyaletinin Maden ve Petrol Bakanlığı, Jeolojik Araştırma Birimi tarafından sağlanan 2D Yulleroo 2007 adlı çok kanallı sismik yansıma projesine ait 3 adet hatta toplanan çok-kanallı sismik yansıma verileri ve Yulleroo-1 ile Yulleroo-2 kuyu bilgileri kullanılarak, Batı Avustralya eyaletinin kuzeybatısında yer alan Canning havzasındaki Fitzroy havzasının batı kesiminin jeolojik yapısı, stratigrafisi ve gaz potansiyeline dair delilleri araştırılmıştır. Çok kanallı sismik yansıma verileri 2007 yılında Terrex Seismic şirketi tarafından toplam 13 hatta yaklaşık 373 km uzunlıkta toplanmıştır. Bu tez kapsamında, Fitzroy havzasında 1967 yılında açılmış olan Yulleroo-1 ve 2008 yılında açılmış olan Yulleroo-2 kuyularına en yakın toplam 83,5 km olan 3 sismik yansıma hattı seçilmiş ve veri işlemleri gerçekleştirilmiştir. Kaynak olarak vibroseis kullanılarak toplanan verilerin uzunluğu 6s, örnekleme aralığı 2 ms olup, Nyquist frekansı 250 Hz’dir. Kayıt esnasında 400 kanal kullanılmış, her bir kanal 20 m aralıklarla dizilmiş olan istasyonlara yerleştirilmiştir. Atış-alıcı arasındaki en yakın mesafe 10 m ve en uzak mesafe ise 3990 m’dir. Atışlar her 20 m de bir tekrarlanarak hatlar tamamlanmıştır. Sismik veriler, Globe Claritas 6.1.1 veri işlem programı kullanılarak veri işlem aşamalarından geçirilmiştir. SEGD formatında toplanan veri öncelikle dahili formata çevirilmiştir. Veri işlem aşamasında geleneksel yol izlenmiş olup bunlar sırasıyla; veriden istenmeyen izlerin ayıklanması, atış-alıcı geometrisinin tanımlanması, faz dönüşümü, statik düzeltme, genlik kazanımı uygulaması, yüzey dalgalarının bastırılması, ters evrişim, frekans ortamında süzgeçleme, ortak derinlik noktası (ODN) gruplaması, normal kayma zamanı (NKZ) düzeltmesi ve hız analizi, yığma işlemi, artık statik düzeltmesi ve sismik göç (migrasyon) ile son olarak otomatik genlik kazancı (AGC)’dir. Sismik yansıma verilerine veri-işlem uygulanarak, düşey ekseni seyahat zamanı olan sismik migrasyon kesitleri elde edilmiştir. Derinlik ortamında yorumlama yapmak amacıyla jeolojik birimler ve zamanlar, kuyu ve stratigrafik bilgiler göz önüne alınarak sismik migrasyon kesitlerinde tespit edilmeye çalışılmıştır. Değerlendirilen sismik yansıma kesitlerinde Yulleroo antiklinali, Canning havzasının oluşumunda önemli etkisi olan Fenton fayı ve yapısal haritalarda gösterilmemiş olan birçok fay gözlenmektedir. Kuyularda belirtilen doğal gaz seviyeleri, sismik yansıma kesitlerinde seçilmiştir. Ayrıca, sismik yansıma kesitlerinden anlık genlik ve anlık frekans kesitleri elde edilerek farklı seviyelerdeki hidrokarbon potansiyeli araştırılmıştır.

In this thesis, the Fitzroy Basin which is located within the Canning Basin in the northwest region of Western Australia (WA), was explored in regards to geological structure, stratigraphy and the probability of the presence of petroleum and gas. This involved processing and interpretation using geophysical methods such as 2D seismic reflection. These methods were applied to the Yulleroo 2D multi-channel seismic reflection data collected in 2007 and was supplemented by the Yulleroo-1 and Yulleroo-2 well logs. This data was kindly provided by the Geological Research Unit in the Department of Mining and Petroleum of Western Australia. In total, data acquisition was performed previously for about 373 km, made up of 13 individual seismic reflection lines, by Terrex Seismic in 2007. For this thesis, three of the seismic reflection lines in the Fitzroy Basin were chosen to be processed and interpreted. They were selected specifically for their proximity to the Yulleroo-1 well, which was drilled in 1967, and the Yulleroo-2 well which was drilled in 2008. The seismic data for these lines were collected using vibroseis as the source, with six second length and a 2ms sample rate and thus had a Nyquest frequency of 250 Hz. Each station consisted of 400 channels with a 20 m gap to record seismic data. Thus, the near-offset between source and receiver was 10 m and the far-offset was 3990 m. For this seismic reflection data acquisition, a group interval of 20m was used. The seismic reflection lines were processed using the Globe Claritas 6.1.1 software package on PC. Initially the SEGD formatted data was converted to the Claritas internal format before processing. This was followed by conventional seismic data processing including: noisy signal elimination; assignment of shot and receiver positions to the headers (geometry); phase conversion; static correction; gain recovery; suppression of ground-roll effects; deconvolution; filtering in the frequency domain; grouping common deep point (CDP); normal moveout (NMO) correction and velocity analysis; seismic stack; residual static correction; seismic migration, and automatic gain control (AGC). Seismic data was collected in the time domain. To be able to interpret in the depth domain, an attempt was made to convert the seismic reflection lines by considering both well logs and stratigraphic information. The seismic lines intersected the Fenton fault which has had a significant effect on the structure of the Canning Basin. Furthermore, the activity of this fault has also caused significant structural changes to the Yulleroo anticlinal via seismic activity. The processed seismic reflection lines showed the Yulleroo anticlinal, Fenton fault and other small faults which current geologic structural maps do not currently show. As a further step, gas levels recorded in the well logs were used to aid location estimates on the seismic lines. In addition, instantaneous amplitude and frequency lines were produced from the seismic data to estimate potential hydrocarbon locations. In previous studies, the Fenton fault, which was found to be a listric fault, and the Yulleroo anticlinal were investigated in multiple seismic reflection sections. The interpretation in this thesis concentrated on the areas around this fault and the anticlinal. In particular, interpretation was performed on three seismic sections which corresponded with the three seismic lines. In the westernmost seismic reflection section, the Fenton fault was detected spanning the time interval from the surface to approximately 2500 ms. Within the same section, the Yulleroo anticlinal was also partially shown. The area of the anticlinal that was most eroded, which is thought to belong to the Jurassic age, was present until 600 ms within this seismic reflection section. It appeared that this eroded area was less deformed above the anticlinal. In the central seismic reflection section, the Fenton fault was detected at an estimated depth between 0 ms and 2400 ms. Within this seismic reflection section, in addition to the entire Yulleroo anticlinal being present, further minor previously unmapped faults were located and marked on the structural geology maps. In the easternmost seismic reflection section, the Fenton fault was also located and appeared to be present until an estimated depth of 2300 ms. The findings of this thesis were concentrated on the area corresponding to a depth estimated to be between 400 ms and 2100 ms. Within this section two novel major unmapped faults were described in further detail than previously with their position being indicated on geological structural maps of the area. The Yulleroo anticlinal, which is present in its entirety within the eastern seismic reflection section, was observed to have greater deformation than in the other seismic reflection sections. Within the Yulleroo anticlinal, the extension effect was more readily observed due to the presence of a greater number of faults. Within this section, the size of the eroded area appeared to increase at approximately 500 ms depth. Within all three seismic sections, the stratigraphic units between the Devonian and the Cretaceous eras could easily be followed by simple visual inspection. Within the central seismic section in particular, the stratigraphic units were particularly clear. Therefore, this section was used as a reference to assist with interpretation of the other seismic sections. Interpretation of the stratigraphic layers within this section was performed by referencing the Yullerroo-1 and Yullerroo-2 well logs collected as part of the Western Australian Well Completion Report (1965). Within this report a layer of Broome sandstone deposited within the Cretaceous era was described at the top of the seismic section. This layer was located within the section at an approximate depth of 150 ms. Below this layer, there were sequential deposits of Jarlemai siltstone, the Alexander formation, and Wallal sandstone. These were determined to exist at approximately between 150 ms and 500 ms depth which would correspond to deposition within the Jurassic era. Below these Jurassic units, approximately between 500 ms and 1600 ms, the Grant groups, the Anderson formation and the Yulleroo formation, which belong to the Carboniferous era, were detected within the central seismic reflection section. The interface of the Devonian age was able to be marked on the seismic reflection sections but it was not possible to make further comments or findings about the geological units for this time period due to the data quality at this depth. At the Yulleroo-1 well, it appeared that there was evidence of gas within the Yulleroo formation between 3342-3357 m and again at 3395-3408 m. In addition, at the Yulleroo-1 well it appeared that Unit C of the Anderson formation contained a good source rock. Furthermore, at the Yulleroo-2 well, further evidence of natural gas was found at ten different points between 2855 m and 2980 m. The likelihood of petrochemicals being found in the study area is high due to the properties of the Anderson formation, which was deposited during the Carboniferous era. Previous stratigraphic section findings have suggested that this formation could be made up of reservoir rock. The seismic attributes of potential gas containing areas, as determined by the instantaneous amplitude and frequency, suggested further evidence of the presence of hydrocarbons. The area with the greatest evidence for the presence of hydrocarbons at both the Yulleroo-1 and Yulleroo-2 wells was best represented in the central seismic reflection line between 1150 ms and 1300 ms deep and was situated within the Yulleroo anticlinal. In the well logs, the evidence suggested that the depth of the gas layers fluctuated between 5 m and 20 m. In the seismic reflection section, which had a maximum lateral resolution of 20m, evidence for the potential gas layer could not be detected. In this study, a flat spot was found in an interface in the Anderson formation on the seismic reflection sections. Seismic attributes were applied to this flat spot and the immediate surroundings on the all of the seismic reflection sections. Therefore the instantaneous amplitude section and instantaneous frequency sections were provided for the immediate area around the flat spot. The interface, which was located on the seismic reflection line closest to the Yulleroo-1 well (central), at approximately 980 ms depth, showed high amplitude values within the instantaneous amplitude section. The instantaneous frequency section for the same interface had substantially lower frequency values. These findings, when taken together with the well logs and stratigraphic information, supported the existence of natural gas above the unit C of the Anderson formation.