n February 2011 LUKOIL acquired the Trident Block to explore this exciting part of the deep-water Black Sea gas region. We operate the block with a 90% equity interest with partners PanAtlantic and Romgaz. In October 2015, LUKOIL’s first offshore exploration drilling campaign in EU (European Union) resulted in a large gas discovery with the drilling of the Lira 1X exploratory well (Fig. 1), substantiating the reasons this block was first acquired.
In February 2011 LUKOIL acquired the Trident Block to explore this exciting part of the deep-water Black Sea gas region. We operate the block with a 90% equity interest with partners PanAtlantic and Romgaz. In October 2015, LUKOIL’s first offshore exploration drilling campaign in EU (European Union) resulted in a large gas discovery with the drilling of the Lira 1X exploratory well (Fig. 1), substantiating the reasons this block was first acquired.
No exploration had previously been undertaken in this part of the Black Sea, aside from widely spaced regional seismic data. The border between Romania and Ukraine has been under dispute for many years because of its potentially large reserves of oil and gas. Not only is the area in a proven hydrocarbon basin, it lies within the Paleo-Danube sand fairway, which suggests improved possibility for finding thick, good quality reservoir sandstones. In comparison, other areas offshore Romania previously yielded only small field sizes principally due to the sparsity of reservoir sands. Upon resolution of the dispute in 2009, Romania moved quickly to open the area up by tender, and LUKOIL moved quickly to outbid many of the largest oil companies in the world for this area.
The Lira- 1X well encountered a gas-saturated interval with a net pay of 46 meters, and the subsequent evaluation indicates the field’s reserves could potentially exceed 1 trillion cubic feet of gas. The post drill evaluation also indicated that multiple prospects of similar nature was de-risked by the well, which warranted further studies to further de-risk and mature those prospects and a complete re-assessment of Trident’s potential.
A series of multidisciplinary studies were launched in 2016 to identify additional exploration potential, which were successfully completed in early 2017. This work included the study of the whole core of Lira-1X, the reprocessing and interpretation of 3D seismic data, the acquisition of Controlled Source Electromagnetic (CSEM) survey, and a seabed geochemistry study which had been first proposed by LUKOIL in its tender bid for the block, and well data exchanges were made with operators of the adjacent blocks. Considering that decisions on future drilling and continuation of the license were on the horizon, it was critical to ensure quality results and timely execution of the studies. Continuous collaboration of the teams in Houston, Bucharest and Moscow and with the technology vendors in US, UK, Russia and Romania was an essential element to the success of the results and the completion of the studies on schedule. The CSEM and geochemistry study results corroborated the interpretation results of the reprocessed 3D seismic data, and the results of the Lira well core study significantly improved the ability to characterize these reservoirs.
Reprocessing of 3D Seismic Data
A state-of-the-art reprocessing of 1000 km2 square kilometers 3D seismic data was conducted to integrate the new well data and bring seismic imaging of the prospective areas to the next level and to facilitate detection and characterization of gas-charged reservoirs in EX-30 block. The data was originally acquired and processed in 2012 but advances in processing and the availability of 2 key wells suggested significant improvement in the identification of additional prospects could be achieved by this new work. A wide array of the latest technologies were applied, including advanced signal processing, multiples suppression, noise elimination, high-resolution velocity modelling and anisotropic depth migration. This work, and results of the seismic inversion interpretation and analysis of the gas-saturated reservoir probability cube not only provided the much needed mitigation of risks previously identified, it resulted in the identification of a large number of leads and prospects within Trident, their extend and properties of the target reservoirs, potentially unlocking significant reserves when confirmed by drilling.
Core Studies of the Lira Reservoir Section
The wireline logs suggested that the gas saturated reservoirs encountered by the Lire well were of a thin-bedded nature, but could not be confirmed. Following extensive evaluation, the conclusion was that the wireline methods could not provide an accurate means to evaluate the reservoir properties to the degree the team required. To address this challenge, the whole core was taken from the key reservoir interval. An extensive study was conducted, providing in-depth and detailed results on both the reservoir architecture and the properties of the reservoir. Visual observations of the core confirmed the thinly bedded reservoirs (Fig. 2), which was then followed by a detailed work program for the core’s evaluation. The studies included high resolution photography, routine and special core sub-samples analyses. Ultra-high resolution core descriptions were also conducted, resulting in nearly 9000 lithological units identified over 109 m of core. This work provided crucial information to improving the reservoir characterization and facies mapping necessary for evaluation and planning of deep-water development.
A CSEM survey (Fig. 3) covering an area of 500 km2 was acquired to evaluate electromagnetic properties of the sedimentary section which can help identify the presence of gas-bearing deposits in the subsurface. Surveyed seabed depths ranged from 300 to 1200 meters. Though still in its infancy, and is continuously being improved, this technology has been used in the past by LUKOIL to help utilize the unique attributes of this form of data to high grade large areas under exploration, and the benefits are if there is a correlation of positive anomalies to the subsurface interpretation using seismic data. The data is acquired by towing a submerged transmitting electrode as a source, with a long (2455 meter) streamer for receiving the returned signal. The results of the study were very encouraging, with positive correlations to the subsurface interpretation over some of the most prospective areas within the block.
Seabed Geochemistry Study
The primary aim of this survey was to obtain confirmation that the active petroleum system in the EX-30 block, proved initially at the Lira discovery, is extending across the block area. The geochemical dataset is used to detect and characterize the petroleum systems highlighting prospective areas, reducing risks involved in future exploration.
A seagoing expedition collected 130 seabed samples over a 850 km2 area (Fig. 4). The samples were then analyzed in the laboratory for hydrocarbon gases and liquids, total organic content, and mineral composition of the seabed samples. Potential micro-seeps were identified and located during the sampling. The sampling was collected from a square grid pattern, but also from key mud volcano features suspected to be sites for the escape of hydrocarbon fluids.
The potential presence of hydrocarbons in the shallow sediments of the seafloor were identified. Gas-charged sediments with gas-hydrates were collected at the Flora mud volcano. Gas-hydrates are solid ice-like form of water that contains gas molecules locked in its structure. This result confirmed Flora to be an active hydrocarbon fluid escape seabed feature (Fig. 5), and supports the wider conclusion of the study, and its role in providing additional evidence to support the subsurface interpretation.
Consideration of the Results
Lukoil teams are currently focused on the planning for the possible drilling of the Trinity-1X well. These studies and the technologies used in arriving at their results have mitigated the exploration risks in the block and supports a potentially significant upside beyond the gas volumes of the Lira field. Further exploration efforts are under consideration by the drilling of an exploration well based upon these studies. The next possible well will target multiple reservoir levels, and reservoirs similar to the Lira discovery. If successful, not only will this new well provide more exploration opportunities in the Trident block by confirmation of the interpretation, it will add additional resource volumes to Lira, and provide a clearer forward roadmap to Trident development and production options.
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