Мурманская МИЛЯ. 2018 г. №4.

EXPLORATIONS Fig. 8 Seismic section of AVO attribute (in tercept - gradient product), illustrating anomalous sectors Рис. 8 Сейсмический разрез AVO-атрибута (произведение интерсепта и градиента), иллюстрирующий аномальные участки 58 quisition tests were carried out that showed high information content and efficiency of the technology for detecting and mapping gas lenses in the upper layer of sediments. When conducting drilling works, the maxi­ mum risk is related to the presence of ampli­ tude anomalies on the seismic sections that are presumably confined to the gas-satu­ rated sediments and fractured zones which, probably, serve as gas migration channels up the section. For anomalous zones analy­ sis and identification we used the following factors: - very high reflection amplitudes (more than 10 times higher than the lateral ave­ rage); - high reflection amplitudes (more than 5 times higher than the lateral average); - reflection phase inversion (polarity in­ version); - “deflection” of seismic events under an anomaly due to reduction of velocity values (“velocity effect”); - sharp lateral amplitude reduction unre­ lated to fractures; - high frequency absorption under ano­ malies; - high values of the AVO-attribute which is the product of intercept by gradient; - amplitude attenuation under an anom­ aly; - amplitude confinement to some weak­ ened zones (including fault systems). The sum of all the above-mentioned fac­ tors means a considerable sediment gas content of the upper part of the section. Anomalous zones are mainly characterized by subhorizontal lateral form and small thick­ ness (Fig. 8) [10]. Weight value was deter­ mined for each factor. Assessment was made according to a 10-point scale. After ranking all the components according to the totality of contribution of each factor, a classification of the amplitude anomalies by degree of risk for drilling was made. The integrated risk as­ sessment index was determined on the basis of the sum of all components. As a result, the following classification was proposed: in­ considerable - 0; low - 1-24; average - 25­ 48; high - 49-72. The results of mapping the areas of anomalously high amplitudes of the wave field according to the data of 2D and 3D surveys are given in Fig. 9 [10]. Based on the results of delineation of all the high-amplitude areas, a composite risk map is made where the color scale cor­ responds to the degree of risk during dril­ ling works. Analysis of the above-mentioned seismic data makes it possible to identify the main geological and geophysical factors de­ termining the conditions for penetration of the upper interval and a project well con­ struction. The identified amplitude anomaly zones have different form, thickness and expansion. The results of the recent surveys performed using high resolution seismic technology demonstrate its high efficiency and informa­ tion content for detailed breakdown of the near-surface section with a view to detecting gas lenses and gas hydrate deposits. The li­ censed blocks of Gazprom and Rosneft in the Sea of Okhotsk and the Kara Sea were surveyed. According to the data of high resolution seismic surveys, the gas content in the sec­ tion is observed, which is manifested by high amplitude anomalies. The identified zones Fig. 9 Example of mapping of domains of abnormally high wave field ranges: a - according to 2D survey data; b - according to 3D survey data Рис. 9 Пример картирования областей аномально высоких амплитуд волнового поля: а — по данным 2D-съемки; б — по данным 3D-съемки MURMANSK MILE • 4-2018