Hydrocarbon prospectivity

Geological offshore prospectivity

Gas

In the Northern part of Croatian part of the Adriatic, 22 gas discoveries were found with estimated reserves of approximately 1.3 trillion cubic feet. Currently, production is established from 19 gas production platforms, one compression platform and almost 51 wells. Annual production amounts approximately 1,2 billion cubic meter of gas.

In Italian part of the Adriatic Sea ,130 gas fields were discovered with reserves estimation of approximately 23 trillion cubic feet. Currently, they are producing from 45 fields with 197 exploitation platforms and almost 600 exploitation wells. Annual production is about 5 billion cubic meters of gas (Ministero dello Sviluppo Economico).

Oil

In the Croatian part of the Adriatic Sea no commercial discovery has been found although two wells, Vlasta-1 and Jadran-13, had significant oil indication but due to commercial and technical reasons exploitation has not been established.

In Italian part of Adriatic Sea, 24 oil discoveries have been found with estimated reserves of 550 MM bbl. Currently, seven oil fields are in the production which are producing from 7 exploitation platform and 39 wells. Annual production amounts approximately 1,8 MM bbl (Ministero dello Sviluppo Economico).

Comparison of exploration and exploitation of hydrocarbons in Croatian and in Italian part of the Adriatic sea

Comparison of exploration and exploitation of hydrocarbons in Croatian and in Italian part of the Adriatic sea

Hydrocarbon prospectivity

Several potential hydrocarbon plays have been identified, based on processed newly acquired 2D seismic data, which could lead to the new discovery of hydrocarbons after further additional seismic acquisition, interpretation and drilling of wells. Examples of selected newly recorded 2D seismic lines are below presented:

Potential gas discoveries

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation and gas discovery ksenija-1 which has not been developed

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation and gas discovery ksenija-1 which has not been developed

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation

New 2d seismic line located in northern Adriatic which indicates potential gas accumulation

Potential oil discoveries

Part of new 2d seismic line located in southern Adriatic which indicates numerous potential accumulation very similar to italian carbonate oil accumulation

Part of new 2d seismic line located in southern Adriatic which indicates numerous potential accumulation very similar to italian carbonate oil accumulation

Part of new 2d seismic line located in southern Adriatic which indicates numerous accumulation on several locations

Part of new 2d seismic line located in southern Adriatic which indicates numerous accumulation on several locations

New 2d seismic line which indicates potential oil accumulation along edge of carbonate platform

New 2d seismic line which indicates potential oil accumulation along edge of carbonate platform

Hydrocarbon prospectivity onshore

Considering that hydrocarbons have been proven and are being actively produced in every region of Pannonian basin onshore Croatia and uncommercial quantities of hydrocarbons have been discovered in the Dinarides prove a working deep hydrocarbon system that is under explored in the region.

The proven hydrocarbon system of the Croatian Pannonian basin contains multiple source reservoir and sealing rocks. In general the main source rocks are composed of:

  • The deltaic sequences of the Pannonian system. Lower sequence deposited in deep-sea and is considered a very good source for oil and gas. The upper deltaic complex is rather gas-prone and it is suggested that it has never been buried deeply enough to reach oil window.
  • Late Neogene source rocks with type II and III kerogen content were deposited as marine or lacustrine clays and marls deposited in restricted lagoonal settings, TOC average 1–2 wt.% (up to 5 wt.%).
  • Early Miocene siltstones and mudstones with kerogen type III, are mature to over-mature and produce dry, wet and gas condensate.
  • Upper Triassic Kössen black shales and marls are considered the principal sources of oil and natural gas in the deep systems.
  • The major proven and producing reservoirs in the basin include:
  • Deep Triassic – Cretaceous limestones and dolomites with fracture and vuggy porosity Neogene series of the Pannonian Basin, all porous sands within the succession may form reservoir units given good structural/stratigraphic position.
  • Miocene-age, near-basement basal clastic bodies (conglomerate). The series represent an excellent reservoir (porosity values may reach up to 61%).
  • Oligocene sands within the “Paleogene basins” (e.g. turbidites of the Kiscell Formation).
  • Fractured metamorphic and igneous basement rocks of Precambrian-Palaeozoic age.

Seals are generally regional, provided by intra-formational pro-delta and delta-top mudstones. In majority of the fields, the argillaceous interval above the reservoir usually forms the sealing horizon. It may also be the source rock for the overlying reservoir. The Neogene series are considered as top seal to all older reservoirs.

There are five main groups of plays discerned in the Pannonian Basin, best classified according to their structural affiliation (majority of fields involve reservoirs at many stratigraphic levels):

  • Miocene-Pliocene clastic and/or carbonate series draped over basement highs form the most significant play types in the basin (such play types are represented by post-rift compactional drape varieties).
  • Fractured and/or weathered basement (highs) with or without Miocene basal conglomerate and sealed by Miocene mudstones form a very important group of plays. (Play types 1 and 2 are occasionally found together).
  • Miocene extension-related, tilted fault blocks containing reservoirs of various ages form an important group of plays (Pre-rift/Basement compactional drape).
  • Inversion-related play types are found in the anticlines associated with reactivation of strike-slip faults and include Miocene-Pliocene clastic reservoirs.
  • Syn-rift-related, Miocene sandstone series truncated and sealed by post-rift mudstones (this play type formally may belong to type 1 plays).

Preliminary analysis of the seismic and other available data shows that the Pannonian contains significant remaining potential and is under explored for unconventional resources. A simple comparison of Hungary and Croatia illustrates that whilst both have similar sized onshore area in the Pannonian, Croatia has only 33% of the number of exploratory wells drilled onshore compared to Hungary (derived from IHS database statistics) and less than 52% of the recoverable reserves discovered to date.

The main undiscovered resources potential is expected to be in the following:

  • Unexplored deeper sections of the Mesozoic
  • Intra-flysch or sub-flysch basins, prospective in Poland and the Ukraine
  • Neogene sub-basin stratigraphic traps
  • Unconventional resource plays (tight gas sands or shale gas) in the L-U Miocene Drava sub-basin or L. Miocene Mura sub-basin
  • Any other areas which are underexplored

In addition to undiscovered resource potential, according to the IHS fields database, there is currently approximately 65 Mmboe recoverable reserves which are undeveloped.

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Željka Rukavina