High-resolution seismic velocity analysis by sign-based weighted semblance

Building an accurate velocity model plays a vital role in routine seismic imaging workflows. Normal-moveout-based seismic velocity analysis is a popular method to make the velocity models. However, traditional velocity analysis methodologies are not generally capable of handling amplitude variations across moveout curves, specifically polarity reversals caused by amplitude-versus-offset anomalies. I present a normal-moveout-based velocity analysis approach that circumvents this shortcoming by modifying the conventional semblance function to include polarity and amplitude correction terms computed using correlation coefficients of seismic traces in the velocity analysis scanning window with a reference trace. Thus, the proposed workflow is suitable for any class of amplitude-versus-offset effects. The approach is demonstrated to four synthetic data examples of different conditions and a field data consisting a common-midpoint gather. Lateral resolution enhancement using the proposed workflow is evaluated by comparison between the results from the workflow and the results obtained by the application of conventional semblance and three semblance-based velocity analysis algorithms developed to circumvent the challenges associated with amplitude variations across moveout curves, caused by seismic attenuation and class II amplitude-versus-offset anomalies. According to the obtained results, the proposed workflow is superior to all the presented workflows in handling such anomalies.

Angle-domain common-image gathers from plane-wave least-squares reverse time migration

Angle-domain common-image gathers (ADCIGs) that can be used for migration velocity analysis and amplitude versus angle analysis are important for seismic exploration. However, because of limited acquisition geometry and seismic frequency band, the ADCIGs extracted by reverse time migration (RTM) suffer from illumination gaps, migration artifacts, and low resolution. We have developed a reflection angle-domain pseudo-extended plane-wave least-squares RTM method for obtaining high-quality ADCIGs. We build the mapping relations between the ADCIGs and the plane-wave sections using an angle-domain pseudo-extended Born modeling operator and an adjoint operator, based on which we formulate the extraction of ADCIGs as an inverse problem. The inverse problem is iteratively solved by a preconditioned stochastic conjugate gradient method, allowing for reduction in computational cost by migrating only a subset instead of the whole dataset and improving image quality thanks to preconditioners. Numerical tests on synthetic and field data verify that the proposed method can compensate for illumination gaps, suppress migration artifacts, and improve resolution of the ADCIGs and the stacked images. Therefore, compared with RTM, the proposed method provides a more reliable input for migration velocity analysis and amplitude versus angle analysis. Moreover, it also provides much better stacked images for seismic interpretation.

Hydrocarbon Mapping on Reservoir Carbonate Using AVO Inversion Method

Amplitude Versus Offset (AVO) inversion has been applied for reservoir analysis focused on the horizon carbonate Peutu and Belumai. Simultaneous inversion analysis is used to determine gas anomaly inside carbonate-rocks and it’s spread laterally around target zones. It is based on the fact that small Vpand Vs value changes are going to show the better anomaly to identify reservoir fluid content. The AVO inversion method applies angle gather data as the input and then it is inverted to produce P impedance (Zp) and S impedance (Zs). Zp and Zs are derived to produce Lambda-Rho and Mu-Rho that are sensitive to fluid and lithology. Value of Mu-Rho between 44–65 Gpa gr/cc while value of Lambda-Rho smaller than 10 Gpa gr/cc (for carbonate-rock filled by fluid). This research found that Lambda-Rho is the best parameter to show the existence of hydrocarbon in the case of gas. While Mu-Rho is the best parameter to show the differences in lithology.

Reexamination of CSEM inversion at the Lona Prospect, Orphan Basin Canada

In 2010 an exploration well was drilled in the Lona prospect in the Orphan Basin, Canada, whose location was based primarily on a structural high at the base of a Cretaceous unconformity. Additionally, three-dimensional (3D) inversion of controlled-source electromagnetic (CSEM) data collected in 2007 and 2009 indicated elevated resistivities that roughly correlated with the structural high, indicating the possibility of hydrocarbon-saturated sediments. The well did not encounter hydrocarbons and the results were considered a false positive. In an effort to better understand the geological significance of the elevated resistivity anomaly, we re-examine the original interpretation of the Lona prospect by using a seismic image-guided inversion algorithm to analyze CSEM data from both a synthetic model and the field data and correlate those results with lithology predictions from seismic amplitude versus angle inversions (AVA). We conclude that the anomalously high resistivities seen are due to high volumes of cemented sand lithology and not to the presence of hydrocarbons. We speculate that if the results shown here had been available at the time, CSEM would not have provided support to the decision to drill.

Fault Related Stress and Fractures Analysis using the Anisotropy Signatures from Azimuthal Amplitude Variation in Lematang Trend, South Sumatera Basin

Evaluasi reservoir terrekahkan harus didasari pengetahuan tentang arah rekah dominan dan stress horizontal yang berlaku pada area studi demi mendapatkan rekomendasi arah pengeboran yang optimal. Namun demikian, informasi tentang stress yang banyak ditemukan adalah pada skala regional, global, maupun skala sumuran. Dalam hal pemodelan pada skala reservoir, data seismic dapat dijadikan penghubung kedua skala yang berjauhan tersebut. Kehadiran rekahan dan stress pada batuan dapat menyebabkan karakter anisotropi di reservoir. Pada skala seismic, sifat anisotropi sebuah media dapat diamati dari perubahan amplitude dan waktu tempuh terhadap azimuth rambat dan pantulan gelombang seismic. Pada makalah ini, kami menggunakan data seismic pre-stack selangkah di depan metode Amplitude Versus Offset (AVO) dan Extended Elastic Impedance (EEI) yang masih mengandalkan perubahan amplitude terhadap sudut pantul, dengan mengamati perubahan amplitude terhadap azimuth (AVAZ). Untuk pencapai tujuan tersebut, kami memproses ulang data seismic 3D dengan menggunakan metode Offset Vector Tile (OVT) sehingga informasi azimuth tersimpan dengan baik. Informasi yang digunakan untuk menginterpretasi orientasi rewkahan dan tress horizontal maximum adalah orientasi dan magnitude anisotropi. Hasil studi ini menunjukkan rekahan dan stress yang bersifat local dapat dengan yang bersifat regional dan global. Hal ini menunjukkan bahwa arah pengeboran optimal tidak harus dikontrol oleh arah stress global.