Title to our oil and natural gas properties is subject to royalty  overriding royalty  carried and other similar interests and contractual arrangements customary in the oil and gas industry  liens incident to operating agreements and for current taxes not yet due and other minor encumbrances. All of our oil and natural gas properties are pledged as collateral under our bank credit facility. As is customary in the oil and gas industry  we are generally able to retain our ownership interest in undeveloped acreage by production of existing wells  by drilling activity which establishes commercial reserves sufficient to maintain the lease  by payment of delay rentals or by the exercise of contractual extension rights. We anticipate retaining ownership of a substantial amount of the acreage with primary terms expiring in 2014 through drilling activity or by extending the leases. The market for our production of oil and natural gas depends on factors beyond our control  including the extent of domestic production and imports of oil and natural gas  the proximity and capacity of natural gas pipelines and other transportation facilities  demand for oil and natural gas  the marketing of competitive fuels and the effects of state and federal regulation. The oil and gas industry also competes with other industries in supplying the energy and fuel requirements of industrial  commercial and individual consumers. Our oil production is currently sold under short-term contracts with a duration of six months or less. The contracts require the purchasers to purchase the amount of oil production that is available at prices tied to the spot oil markets. Our natural gas production is primarily sold under contracts with various terms and priced on first of the month index prices or on daily spot market prices. Approximately 87% of our 2013 natural gas sales were priced utilizing first of the month index prices and approximately 13% were priced utilizing daily spot prices. BP Energy Company and its subsidiaries and Shell Oil Company and its subsidiaries accounted for 51% and 36%  respectively  of our total 2013 sales. The loss of either of these customers would not have a material adverse effect on us as there is an available market for our crude oil and natural gas production from other purchasers. We have entered into longer term marketing arrangements to ensure that we have adequate transportation to get our natural gas production in North Louisiana to the markets. As an alternative to constructing our own gathering and treating facilities  we have entered into a variety of gathering and treating agreements with midstream companies to transport our natural gas to the long-haul natural gas pipelines. We have entered into certain agreements with a major natural gas marketing company to provide us with firm transportation for 55 000 MMBtus per day for our North Louisiana natural gas production on the long-haul pipelines. These agreements expire from 2015 to 2019. To the extent we are not able to deliver the contracted natural gas volumes  we may be responsible for the transportation costs. Our production available to deliver under these agreements in North Louisiana is expected to exceed the firm transportation arrangements we have in place. In addition  the marketing company managing the firm transportation is required to use reasonable efforts to supplement our deliveries should we have a shortfall during the term of the agreements. The oil and gas industry is highly competitive. Competitors include major oil companies  other independent energy companies and individual producers and operators  many of which have financial resources  personnel and facilities substantially greater than we do. We face intense competition for the acquisition of oil and natural gas properties and leases for oil and gas exploration. 20 Competition Markets and Customers